docs: record contract-monitor automation flow

[CodeSigils]

Project

Docs update — README.md, CONTRIBUTING.md, and proposals/ROADMAP.md. No list entry or skill payload change.

Why it belongs

The repository docs still described the pre-#106 contract-monitor flow: "opens a reviewable PR for every drift fix (including timestamp-only refreshes)" and "required PR checks are intentionally relaxed during solo evaluation."

The live behavior after PRs #106–#109 is different and now documented here:

• Timestamp-only refreshes — the monitor opens a peter-evans PR and the workflow merges it directly with gh pr merge <n> --squash --delete-branch (signed, GitHub-verified squash). No human step; no PR backlog (this resolves the chore: auto-fix URL drift detected by contract monitor #102/chore: auto-fix URL drift detected by contract monitor #105 stuck-bot-PR loop; chore: auto-fix URL drift detected by contract monitor #105 was closed as superseded).
• Canonical URL corrections — still open a reviewable PR, now labeled needs:semantic-review, merged by a maintainer only.
• Why no auto-merge — main has zero required status checks (protection is required_signatures: true, no rulesets, no required checks, no reviews), so GitHub declines --auto on an already-clean PR (Pull request is in clean status (enablePullRequestAutoMerge)). Direct merge is the correct mechanism here.

Quality evidence

• Wording mirrors the merged behavior verified live: chore: split contract-monitor auto-fix by drift class #106 (class split), chore: auto-merge timestamp-only contract-monitor refreshes #107 (pivot to PR + auto-merge attempt), chore: auto-fix URL drift detected by contract monitor #108 (direct merge proven, 81e4e4a), chore: merge timestamp-only contract-monitor PR directly #109 (workflow step switched from --auto to direct --squash --delete-branch, 5c70217).
• Docs-only change (3 files, +28/-13); no workflow, script, or state-file edits; SKILL.md left untouched because its "Repository CI monitors the evidence sources" statement plus runtime-freshness caveat remains accurate.

Checklist

• Single logical change (contract-monitor docs alignment) in one PR
• Read CONTRIBUTING.md and CRITERIA.md before editing
• Entry/format conventions respected (no list entries touched)
• Alphabetical ordering unaffected (not a list change)
• Description factual, matches merged behavior, not marketing
• Not a tutorial, sample, or commercial material
• No advisory signals or baselines edited
• Did not edit advisory-baseline.json / repo-exceptions.json
• Local checks equivalent performed: docs-only, no test impact
• Maintenance directive followed: ROADMAP implementation-record entry + revision consistency