Fix review findings: validate the hooks cap, integer read caps, doc drift, config plumb-through test

A Codex review pass on the draft caught four real gaps and two solid
suggestions; all addressed except one pushed back on the merits:

- hooks-claude/hooks-codex: stderrSummaryMaxChars was the one new knob
  with NO range validation — a negative/NaN cap would silently
  misbehave inside slice(). Both bridges now assert a positive integer
  at the TOP of apply() (before the config-file parse's early return,
  so a bad value fails the load loudly), with rejection tests.
- tool-fs: the read caps count lines/chars/bytes, so positive-FINITE
  was too loose (a fractional readLimit would flow into windowing
  arithmetic and the schema description). All four now require a
  positive integer, matching tool-web's cap.
- Doc drift the gates cannot catch: tool-web's README tools table
  still named WEB_SEARCH_MAX_RESULTS as the mechanism; compact-basic's
  README/module doc and the compaction-capability-seam RFC still
  described estimation as fixed char/4 rather than the charsPerToken
  default.
- subagent-acp: the dispose graces were tested only at the
  startAcpRun level, so a regression that stopped threading plugin
  config into AcpRunSpec would have survived. A provider-path test now
  drives the trap-escalation scenario through ctx.subagents.start with
  small config graces and bounds dispose at 4s.

Pushed back on: converting compact-basic's charsPerToken to a
schemastery field. The package's whole config is deliberately
hand-rolled (resolveConfig, every threshold REQUIRED with no default —
a documented design posture); one schemastery field beside it would be
incoherent. The knob is cordis.yml-reachable, defaulted, and validated,
which is what the convention requires; migrating the package to
schemastery wholesale is pre-existing config-surface hygiene out of
this change's scope.

packages/compact/compact-basic/README.md

@@ -1,6 +1,6 @@
 # @deepseek-ai/dsh-compact-basic
 
-The **basic compaction backend**: a `BasicCompactService` implementing the `@deepseek-ai/dsh-compact` seam with a char/4 token heuristic, token-budget retention, and summarization routed through the agent request pipeline.
+The **basic compaction backend**: a `BasicCompactService` implementing the `@deepseek-ai/dsh-compact` seam with a chars-per-token heuristic (the `charsPerToken` config, default 4), token-budget retention, and summarization routed through the agent request pipeline.
 
 This is the implementation tier of the compaction capability — see the [interface package](../compact/README.md) for the seam and the [capability-seam RFC](../../../docs/rfc/implemented/feature/2026-06-18-compaction-capability-seam.md) for the design.
 
@@ -8,7 +8,7 @@ This is the implementation tier of the compaction capability — see the [interf
 
 The abstract contract states only WHAT compaction does; this backend owns every HOW decision:
 
-- **Token estimation** — `estimateContentTokens()`: char/4 with per-block structural overhead (`text`/`reasoning` = `ceil(len/4) + 4`, `tool-call` from name + arguments, `tool-result` recursive, `image` = 85, unknown blocks via JSON length).
+- **Token estimation** — `estimateContentTokens()`: chars divided by the `charsPerToken` config (default 4) with per-block structural overhead (`text`/`reasoning` = `ceil(len/charsPerToken) + 4`, `tool-call` from name + arguments, `tool-result` recursive, `image` = 85, unknown blocks via JSON length).
 - **Retention policy** — `compactIfNeeded()` walks the surface nodes tail→head summing per-node token estimates, and retains the smallest tail-run of WHOLE units (a closed step, or a single no-step node such as a pre-step `user/message` or inter-step `steering/message`) whose total reaches `retainTokens`; everything older is compacted. Retention is **turn-agnostic** — turn boundaries play no role, so a single runaway turn that alone exceeds the window compacts its OWN early closed steps rather than being retained verbatim (the failure mode that motivated dropping turn-protection: a tool-heavy turn must stay compactable or the harness dies exactly when compaction is needed). The only structural guard is **tool-pairing balance**: the compacted region's edges are balanced cuts on the surface (no unanswered tool-call crosses either edge), so it never splits a step's `assistant/message` tool-calls from their `tool/result`s. When the only compactable content left is an un-splittable open tail step, it declines (returns `null`) and retries once an older step closes. **Single-unit overflow is out of scope, by design**: if one retained unit (a single closed step, or a large pasted `user/message`) ALONE exceeds the budget, compaction cannot help and the call may go out over-budget — bounding an individual unit's size is a separate concern. `compactRegion()` enforces tool-pairing balance strictly, throwing on a boundary that would split a step. `dsh-session` exports `isToolPairingBalanced` for the check.
 - **Dynamic convergence** — no static summary-length config pretends to bound what the model will write. If framing/estimator/system overhead leaves the compacted surface above threshold, `compactIfNeeded()` re-compacts the head checkpoint up to `compactionRetries` extra times; if it still cannot get below threshold, it throws. A summary whose estimated stored size is not smaller than the shadowed content fails closed before it mutates the surface.
 - **Summarization** — `summarize()`: a `GenerateOptions` request assembled via `BlockAssembler` with a fixed system prompt that asks for a structured checkpoint (Primary Request and Intent · Key Technical Concepts · Files and Code · Errors and Fixes · Pending Tasks · Current Work · Next Step · Critical Context), every section mandatory, exact paths/commands/identifiers preserved. The request runs through the `agent/request` waterfall before `ctx.llm.stream()`, so router agents that choose the concrete model there also route compaction summaries. `maxTokens` is the provider-side generation cap; only text blocks from the model's reply are kept before the checkpoint is stored (reasoning is dropped so private chain-of-thought never leaks into the durable summary, and a stray `tool-call` is dropped so the synthesized `user/message` summary cannot land an orphaned call with no matching `tool-result`). The compacted region is flattened to a plain-text transcript first: text and reasoning contribute their text, and every non-text block (image, tool-call, tool-result, plugin-added types) contributes a type-tagged placeholder (`[image]`, `[tool-call: name(args)]`, …) so the summarizer is told what existed rather than silently dropping it.