Expose audited hardcoded tunables as plugin config

The audit swept every packages/*/* plugin for the new AGENTS.md
convention (no hardcoded tunables in plugins) and exposes each finding
as a defaulted, validated Config field. Defaults are the previously
hardcoded values throughout, so no deployment or golden changes.

- tool-fs (had NO Config): readLimit, readMaxLineLength, readMaxBytes,
  readStreamMinSize. The caps thread through ReadToolCaps/ReadWindow —
  read-render already documented that the consumer applies the caps, so
  they become explicit per-request fields.
- tool-web: searchMaxResults (WEB_SEARCH_MAX_RESULTS stays as the
  schemastery default). Also fixes the stale GREP_LIMIT references in
  search.ts and the web-capability-seam RFC (no such constant exists).
- bash-local: graceMs (SIGTERM->SIGKILL escalation grace). The
  RunInternals.graceMs test seam is gone: graceMs is now a required
  SpawnSpec field filled from config, so tests exercise the real
  config path and the defaults live in exactly one place.
- subagent-acp: disposeEofGraceMs / disposeGraceMs. The AcpRunSpec
  fields become required for the same one-defaulting-layer reason.
- session-persistence-sqlite: journalMode ('wal' default; the
  rollback-journal modes serve filesystems where WAL's shared-memory
  files do not work, e.g. network mounts).
- hooks-claude + hooks-codex: stderrSummaryMaxChars for the persisted
  hook/result stderr summary. The duplicated summarize() helpers merge
  into hook-protocol's summarizeStderr(stderr, maxChars), beside the
  HookResultRecord field it feeds, with the bound parameterized the
  same way runHook's defaultTimeoutMs already is.
- compact-basic: charsPerToken for the token estimator (default 4, the
  English-text heuristic; CJK-heavy deployments need ~1-2 or compaction
  fires far too late). Also corrects the BasicCompactService class doc,
  which claimed defaults the required-field config never had.
- fs-local: deletes the dead STREAM_MIN_SIZE constant and the dead
  FsIoInternals.streamMinSize seam — the read-routing bound lives in
  the consumer (tool-fs), where it is now config. This is item 1 of
  the proposed prune-write-only-fs-surface RFC, annotated accordingly.

Every new field gets range validation (following the existing
assertPositiveFinite pattern), a README row, and tests covering the
configured behavior, the schema default, and load-time rejection.

packages/compact/compact-basic/src/index.ts

@@ -148,9 +148,11 @@ function finishError(finish: FinishReason): Error | undefined {
 }
 
 /**
- * Basic, dependency-light compaction backend. Defaults target a 128K context
- * window, compacting at 80% utilization and retaining ~20K tokens of recent
- * context.
+ * Basic, dependency-light compaction backend: estimates the surface's token
+ * footprint, summarizes the stale prefix through the model, and shadows it
+ * behind a durable checkpoint. Every threshold/budget knob is required config
+ * ({@link BasicCompactConfig}); the estimator's text density is the
+ * `charsPerToken` knob.
  */
 export class BasicCompactService extends CompactService {
   static inject = ['llm']
@@ -207,24 +209,27 @@ export class BasicCompactService extends CompactService {
 
   // ---- Token estimation (overridable hooks) ----
 
-  // TODO: char/4 is a coarse heuristic. Replace with an exact count — a real
-  // tokenizer, or the provider's post-response `usage` (input tokens) fed back
-  // as a correction — so threshold decisions match the model's actual budget.
+  // TODO: chars/charsPerToken is a coarse heuristic. Replace with an exact
+  // count — a real tokenizer, or the provider's post-response `usage` (input
+  // tokens) fed back as a correction — so threshold decisions match the
+  // model's actual budget.
   /**
-   * Estimate the token count of content blocks — char/4 with per-block
-   * overhead. Override in a subclass to plug in a real tokenizer.
+   * Estimate the token count of content blocks — chars divided by the
+   * `charsPerToken` config, with per-block overhead. Override in a subclass to
+   * plug in a real tokenizer.
    */
   estimateContentTokens(blocks: readonly ContentBlock[]): number {
+    const { charsPerToken } = this.config
     let tokens = 0
     for (const block of blocks) {
       switch (block.type) {
         case 'text':
         case 'reasoning':
-          tokens += Math.ceil(block.text.length / 4) + BLOCK_OVERHEAD
+          tokens += Math.ceil(block.text.length / charsPerToken) + BLOCK_OVERHEAD
           break
         case 'tool-call':
-          tokens += Math.ceil(block.name.length / 4)
-            + Math.ceil(block.arguments.length / 4)
+          tokens += Math.ceil(block.name.length / charsPerToken)
+            + Math.ceil(block.arguments.length / charsPerToken)
             + BLOCK_OVERHEAD
           break
         case 'tool-result':
@@ -236,7 +241,7 @@ export class BasicCompactService extends CompactService {
         default:
           // Unknown block types (merge-extensible ContentBlockMap):
           // estimate conservatively via JSON stringify.
-          tokens += BLOCK_OVERHEAD + Math.ceil(JSON.stringify(block).length / 4)
+          tokens += BLOCK_OVERHEAD + Math.ceil(JSON.stringify(block).length / charsPerToken)
       }
     }
     return tokens
@@ -266,7 +271,7 @@ export class BasicCompactService extends CompactService {
       total += this.estimateContentTokens(msg.content)
       total += ROLE_OVERHEAD
     }
-    if (systemPrompt) total += Math.ceil(systemPrompt.length / 4)
+    if (systemPrompt) total += Math.ceil(systemPrompt.length / this.config.charsPerToken)
     return total
   }