fix: Migrate planner, predictor, supervisor to SDK (#6)

Replace tmux-based run_formula_and_monitor() with synchronous agent_run()
from lib/agent-sdk.sh, matching the pattern established in gardener-run.sh.

Key changes per agent:
- Drop agent-session.sh, use agent-sdk.sh (SID_FILE, LOGFILE)
- Remove SESSION_NAME, PHASE_FILE, PHASE_POLL_INTERVAL (tmux/phase artifacts)
- Strip phase protocol from prompt footer (SDK mode needs no phase signals)
- Preserve all prompt composition: context blocks, memory, journal, preflight

Shared helpers added to lib/formula-session.sh:
- build_sdk_prompt_footer(): build_prompt_footer minus phase protocol
- formula_worktree_setup(): fetch + cleanup + create worktree + EXIT trap

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

gardener/gardener-run.sh

@@ -92,11 +92,8 @@ Supported actions:
 The commit-and-pr step converts JSONL to JSON array. The orchestrator executes
 actions after the PR merges. Do NOT call mutation APIs directly during the run."
 
-# Reuse shared footer (API reference + environment), replace phase protocol
-# shellcheck disable=SC2034  # consumed by build_prompt_footer
-PHASE_FILE=""  # not used in SDK mode
-build_prompt_footer "$GARDENER_API_EXTRA"
-PROMPT_FOOTER="${PROMPT_FOOTER%%## Phase protocol*}## Completion protocol (REQUIRED)
+build_sdk_prompt_footer "$GARDENER_API_EXTRA"
+PROMPT_FOOTER="${PROMPT_FOOTER}## Completion protocol (REQUIRED)
 When the commit-and-pr step creates a PR, write the PR number and stop:
   echo \"\$PR_NUMBER\" > '${GARDENER_PR_FILE}'
 Then STOP. Do NOT write PHASE: signals — the orchestrator handles CI, review, and merge.