fix(preamble): echo SESSIONS count so CLAUDE.md rules can read it (#1651 Defect A)

[0xDevNinja]

Summary

• The preamble at scripts/resolvers/preamble/generate-preamble-bash.ts:23 computes _SESSIONS=$(find ~/.gstack/sessions -mmin -120 -type f | wc -l | tr -d ' ') but never echoes it. Every other preamble state value (BRANCH, PROACTIVE, SKILL_PREFIX, REPO_MODE, TELEMETRY, EXPLAIN_LEVEL, QUESTION_TUNING, LEARNINGS, ARTIFACTS_SYNC, ...) is echoed for the calling agent to read; SESSIONS is the lone hole.
• Any CLAUDE.md rule that points at "the preamble's session count" has no value to read and silently ships non-functional, as the reporter caught on a post-ship audit.
• Add echo "SESSIONS: $_SESSIONS" next to the existing BRANCH/PROACTIVE/SKILL_PREFIX block in the preamble, plus an invariant test in test/gen-skill-docs.test.ts that fails if any SKILL.md containing the _SESSIONS=$(find computation no longer echoes the value.

Closes #1651 Defect A only.

Out of scope (Defect B)

The reporter's Defect B — marker mtime falling outside the 2h window for long-running sessions that don't invoke a gstack skill in that span — needs hook-infra work (PreToolUse refresh on every tool call) and is a separate change with broader surface area. Leaving it for a follow-up so this PR stays small and safe to land.

Diff shape

• 1 resolver edit: +1 echo line
• 1 test addition: +30 lines (invariant pinning the echo across all generated SKILL.md files)
• 47 regenerated SKILL.md files (no manual edits; produced by bun run gen:skill-docs)

Tests

bun test test/skill-validation.test.ts test/gen-skill-docs.test.ts
# 712 pass, 7 fail (existing 711 + 1 new invariant)

The 7 failures are the pre-existing --dry-run freshness check cases on non-Claude hosts that reproduce on upstream/main with this branch's changes stashed — unrelated.

Verification

$ grep -n "SESSIONS:" ship/SKILL.md context-save/SKILL.md
ship/SKILL.md:47:echo "SESSIONS: $_SESSIONS"
context-save/SKILL.md:46:echo "SESSIONS: $_SESSIONS"

A user-side rule like

If preamble emits SESSIONS: 2 (or higher), force a worktree switch before any edit.

now has a value to read instead of having to inline the find itself.