fix: Extract lib/pr-lifecycle.sh — walk-PR-to-merge library (#795)

New reusable library with clean function boundaries for the PR lifecycle:
- pr_create, pr_find_by_branch — PR creation and lookup
- pr_poll_ci — poll CI with infra vs code failure classification
- pr_poll_review — poll for review verdict (bot comments + formal reviews)
- pr_merge, pr_is_merged — merge with 405 handling and race detection
- pr_walk_to_merge — full orchestration loop (CI → review → merge)
- build_phase_protocol_prompt — git push instructions for agent prompts

The pr_walk_to_merge function uses agent_run() which callers must define
(guarded stub provided). This bridges to the synchronous SDK architecture
where the orchestrator bash loop IS the state machine — no phase files.

Extracted from: dev/phase-handler.sh, dev/dev-poll.sh, lib/ci-helpers.sh
Smoke test updated to include the new library.

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

.woodpecker/agent-smoke.sh

@@ -113,7 +113,7 @@ echo "=== 2/2  Function resolution ==="
 # If a new lib file is added and sourced by agents, add it to LIB_FUNS below
 # and add a check_script call for it in the lib files section further down.
 LIB_FUNS=$(
-  for f in lib/agent-session.sh lib/env.sh lib/ci-helpers.sh lib/load-project.sh lib/secret-scan.sh lib/file-action-issue.sh lib/formula-session.sh lib/mirrors.sh lib/guard.sh; do
+  for f in lib/agent-session.sh lib/env.sh lib/ci-helpers.sh lib/load-project.sh lib/secret-scan.sh lib/file-action-issue.sh lib/formula-session.sh lib/mirrors.sh lib/guard.sh lib/pr-lifecycle.sh; do
     if [ -f "$f" ]; then get_fns "$f"; fi
   done | sort -u
 )
@@ -186,6 +186,7 @@ check_script lib/formula-session.sh     lib/agent-session.sh
 check_script lib/load-project.sh
 check_script lib/mirrors.sh              lib/env.sh
 check_script lib/guard.sh
+check_script lib/pr-lifecycle.sh
 
 # Standalone lib scripts (not sourced by agents; run directly or as services).
 # Still checked for function resolution against LIB_FUNS + own definitions.