fix: gardener migration — run-gardener.toml via direct cron, remove legacy scripts (#490)

Rewrite gardener-run.sh as direct cron runner (matching supervisor/planner/
predictor pattern): lock guard, memory check, worktree, tmux session with
Claude sonnet + formulas/run-gardener.toml, phase monitoring, cleanup.

- Delete gardener-poll.sh and gardener-agent.sh (superseded)
- Extract consume_escalation_reply() to lib/formula-session.sh (shared
  by gardener and supervisor, eliminates duplicate blocks)
- Update AGENTS.md, gardener/AGENTS.md, lib/AGENTS.md, CI smoke test,
  and cross-references

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

formulas/run-gardener.toml

@@ -1,7 +1,7 @@
 # formulas/run-gardener.toml — Gardener housekeeping formula
 #
 # Defines the gardener's complete run: grooming (Claude session via
-# gardener-agent.sh) + blocked-review + AGENTS.md maintenance + final
+# gardener-run.sh) + blocked-review + AGENTS.md maintenance + final
 # commit-and-pr.
 #
 # No memory, no journal. The gardener does mechanical housekeeping
@@ -48,7 +48,7 @@ id    = "grooming"
 title = "Backlog grooming — triage all open issues"
 description = """
 Groom the open issue backlog. This step is the core Claude-driven analysis
-(currently implemented in gardener-agent.sh with bash pre-checks).
+(Claude performs pre-checks inline before deeper analysis).
 
 Pre-checks (bash, zero tokens — detect problems before invoking Claude):