fix(session-start): support ECC_SESSION_RETENTION_DAYS opt-out + document env var (#2151) (#2163)

* fix(session-start): support ECC_SESSION_RETENTION_DAYS opt-out + document env var

The retention pass for *-session.tmp files (issue #2151) landed previously,
but the env var that controls it was undocumented in the README and rejected
falsy values (0, off, disabled), silently falling back to the 30-day default.
Users who want to keep all sessions for forensic or research workflows had no
way to opt out.

This patch:

- Extends getSessionRetentionDays() so 0|off|false|disabled|never|none disables
  pruning entirely (returns null sentinel; default behavior unchanged).
- Updates the call site in main() to skip pruneExpiredSessions when retention
  is null and emits a clear "[SessionStart] Pruning disabled via
  ECC_SESSION_RETENTION_DAYS" log line so the operator can tell pruning is off.
- Documents ECC_SESSION_RETENTION_DAYS in the README "Hook Runtime Controls"
  section alongside the other ECC_SESSION_* knobs.
- Adds three regression tests in tests/hooks/hooks.test.js covering opt-out
  via 0, opt-out via off, and garbage-value fallback to default 30.

Verification:
- node tests/hooks/hooks.test.js  — 240/240 green (incl. 3 new retention tests)
- node tests/run-all.js           — 2622/2622 green
- npx eslint scripts/hooks/session-start.js tests/hooks/hooks.test.js — clean
- node scripts/ci/validate-no-personal-paths.js — clean
- node scripts/ci/check-unicode-safety.js       — clean
- node scripts/ci/validate-hooks.js — 28 matchers validated
- node scripts/ci/validate-rules.js — 115 files validated

Fixes #2151

* docs(readme): list all ECC_SESSION_RETENTION_DAYS opt-out values + add Windows example

Address reviewer feedback on PR #2163:
- CodeRabbit and cubic both flagged that the README docs only listed 3 of 6
  opt-out values accepted by getSessionRetentionDays() (0, off, disabled),
  while the implementation also accepts false, never, none.
- cubic also flagged the missing Windows PowerShell example for the new
  variable, breaking the parallel structure of the existing
  ECC_CONTEXT_MONITOR_COST_WARNINGS example block.

Updated the README to:
- Spell out all six opt-out values (0, off, false, disabled, never, none)
  and clarify they "keep all sessions (disable pruning)".
- Add an ECC_SESSION_RETENTION_DAYS line to the Windows PowerShell example.

No behavior change. README only.

Verification:
- npx markdownlint README.md — clean
- npx eslint scripts/hooks/session-start.js tests/hooks/hooks.test.js — clean

scripts/hooks/session-start.js

@@ -83,9 +83,22 @@ function dedupeRecentSessions(searchDirs) {
     .sort((left, right) => right.mtime - left.mtime || left.dirIndex - right.dirIndex);
 }
 
+/**
+ * Resolve session retention days from the ECC_SESSION_RETENTION_DAYS env var.
+ *
+ * @returns {number|null} The retention window in days, or `null` when the
+ *   user has explicitly opted out of pruning. Falsy/garbage values fall back
+ *   to {@link DEFAULT_SESSION_RETENTION_DAYS}.
+ */
 function getSessionRetentionDays() {
   const raw = process.env.ECC_SESSION_RETENTION_DAYS;
   if (!raw) return DEFAULT_SESSION_RETENTION_DAYS;
+
+  const normalized = String(raw).trim().toLowerCase();
+  if (['0', 'off', 'false', 'disabled', 'never', 'none'].includes(normalized)) {
+    return null;
+  }
+
   const parsed = Number.parseInt(raw, 10);
   return Number.isInteger(parsed) && parsed > 0 ? parsed : DEFAULT_SESSION_RETENTION_DAYS;
 }
@@ -526,9 +539,13 @@ async function main() {
   ensureDir(learnedDir);
 
   const retentionDays = getSessionRetentionDays();
-  const prunedSessions = pruneExpiredSessions(sessionSearchDirs, retentionDays);
-  if (prunedSessions > 0) {
-    log(`[SessionStart] Pruned ${prunedSessions} expired session(s) older than ${retentionDays} day(s)`);
+  if (retentionDays === null) {
+    log('[SessionStart] Pruning disabled via ECC_SESSION_RETENTION_DAYS');
+  } else {
+    const prunedSessions = pruneExpiredSessions(sessionSearchDirs, retentionDays);
+    if (prunedSessions > 0) {
+      log(`[SessionStart] Pruned ${prunedSessions} expired session(s) older than ${retentionDays} day(s)`);
+    }
   }
 
   const observerSessionId = resolveSessionId();