fix: persist session state across Bash tool calls for accurate telemetry

[walton-chris]

Summary

The preamble sets _TEL_START and _SESSION_ID as shell variables,
but shell state does not survive between separate Bash tool
invocations (each Bash call is a fresh shell). When the final
## Telemetry (run last) block runs in a new shell, both variables
are empty — the arithmetic evaluates to 0 and the session id falls
back to "unknown".

Net effect: every timeline entry logs duration_s:"0" and
session:"unknown", regardless of how long the skill actually ran.
Session-correlation analytics are currently useless.

Evidence

Apex-coach office-hours session 2026-04-12:

{"skill":"office-hours","event":"started","branch":"master",
 "session":"10269-1775994715","ts":"2026-04-12T11:51:56.513Z"}
{"skill":"office-hours","event":"completed","branch":"master",
 "outcome":"success","duration_s":"0","session":"unknown",
 "ts":"2026-04-12T11:59:40.368Z"}

~8 real minutes, logged as 0. Session ids don't correlate. Every
other completed timeline entry across the install has the same
signature.

Fix

One edit to scripts/resolvers/preamble.ts:

1. After setting _TEL_START and _SESSION_ID, write a small state
   file to ~/.gstack/analytics/.session-state via heredoc.
2. At the top of the telemetry block, source the state file (using
   . not source for POSIX portability).
3. Compute duration with \${_TEL_START:-\$_TEL_END} fallback so
   missing-file scenarios gracefully degrade to the current behavior
   rather than erroring.
4. Remove the state file after telemetry fires.

All SKILL.md files regenerated via bun run gen:skill-docs --host all.
Golden-file fixtures (claude/codex/factory ship) updated.

Scope: preamble-tier 3 skills pick up the fix via regeneration.
Minimal-preamble skills (freeze, guard, careful, unfreeze,
gstack-upgrade) don't include the telemetry block at all and are
unaffected.

Test plan

• Run any skill, confirm ~/.gstack/analytics/.session-state
  is written during preamble
• Confirm duration_s in the skill's timeline.jsonl event
  is a non-zero number matching real session duration
• Confirm session is the PID-timestamp format set in preamble
  (not "unknown")
• Delete the state file mid-session, confirm telemetry degrades
  gracefully (duration 0, no crash)
• Confirm state file is cleaned up after the skill completes

Relationship to #1049

PR #1049 (office-hours artifact verification) is orthogonal but
complementary. Once both land, office-hours correctly tracks duration
AND correctly distinguishes successful sessions (artifact written)
from silent failures (no artifact). This PR is generic; that one is
office-hours-specific.

Commit history on this branch

4 commits. First is the substantive fix; the rest are regen/fixture
updates needed to land the first commit cleanly. Garry's usual
squash-merge pattern will collapse them on merge.

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