refactor: split monitor.go (2051 lines) into 8 focused files

[tzone85]

Summary

• Pure file split of internal/engine/monitor.go (2051 lines) into 8 sibling files under the same package engine.
• No behavioural change: same Monitor struct, same method receivers, same package-level helpers (autoCommit, gitDiff, FindDependents, IsStoryComplete, vxdArtifactPatterns).
• Largest sibling is now 538 lines (monitor_post_execution.go), well under the 800-line house guideline.

Layout

File	Lines	Contents
monitor.go	221	struct + 14 setters + RunContext
monitor_polling.go	133	Run, RunWithContext, pollOnce
monitor_sla.go	167	checkSLA, clearSLATracking, escalateOnSLABreach
monitor_post_execution.go	538	postExecutionPipeline, rebaseAndMerge, rebaseAndCreatePR, pauseRequirement, resetStoryToDraft
monitor_dispatch.go	260	dispatchNextWave, FindDependents, IsStoryComplete
monitor_escalation.go	322	tier-2 manager + tier-3 tech-lead handlers
monitor_git_hygiene.go	363	autoCommit, stripVXDArtifactsFromBranch, pullMainAfterMerge, ensureGitignorePatterns
monitor_gitdiff.go	123	gitDiff*, captureFileTree, artifact patterns

Diff stat: 1907 insertions, 1832 deletions — the 75-line delta is the package engine header + scoped import block repeated across the 8 files (trimmed to only the imports each file actually uses via goimports).

Test plan

• go build ./... clean
• go vet ./internal/engine/ clean
• go test ./... -count=1 — all 30 packages pass
• ~/.local/bin/vxd --version smoke check
• Closes follow-up feat: add cost estimation and pre-flight validation #26 in CLAUDE.md
• Obsidian launch playbook updated to reflect closed item