Check docs conflict markers

[azimafroozeh]

What & why

This PR makes CI fail on committed merge-conflict markers, in two layers.

• Proven escape: rfc: add RFC 0040, the system column namespace #546 squash-merged literal conflict markers into docs/rfcs/README.md on main, where they shipped until docs(rfcs): remove committed merge-conflict markers from the registry #598 removed them. Markers in Rust die in compile; markers in Markdown, fixtures, and data files pass every existing PR check, because no PR gate reads those files.
• Layer 1: scripts/check-docs.py gains a marker scan over every Markdown file it already checks, reporting exact file:line.
• Layer 2: a step in the same job rejects any pull request whose own diff adds a conflict-marker line, in any file type, using git's built-in detector (git diff --check, which respects conflict-marker-size and also flags a lone ======= on added lines). It emits an inline ::error annotation per offending line plus a count, runs before the documentation checks, and never fails a pull request for markers already on the base branch.
• Both ride the existing required Check AGENTS.md Links context, which runs on every PR including documentation-only ones: no new job, no branch-protection change.

Backing issue / RFC

• none; trivial fast-lane: closes the gap docs(rfcs): remove committed merge-conflict markers from the registry #598 noted (check-docs.py did not scan for conflict markers) after the rfc: add RFC 0040, the system column namespace #546 escape.

Checklist

• Change is focused (one scan in check-docs.py, one CI step, their mention in docs/dev/ci.md)
• Tests added/updated for behavior changes (check-docs.py has no test harness; verified red-then-green locally, see Local verification)
• Public docs updated if user-facing surface changed (developer-facing only; docs/dev/ci.md updated)
• Reviewed against docs/dev/invariants.md — no Hard Invariant weakened, no deny-list item hit (docs tooling only, no engine surface)

Local verification

• python3 scripts/check-docs.py: Documentation OK (121 Markdown files checked)
• Red phase: same command with a probe file carrying rfc: add RFC 0040, the system column namespace #546-shaped markers (<<<<<<< HEAD … >>>>>>> 463fd70e (review)) exits 1 with two exact file:line: committed merge-conflict marker errors; probe removed, green again
• The CI step's script body, run verbatim over real history: on the rfc: add RFC 0040, the system column namespace #546 commit (the one that introduced the markers) it emits three ::error file=docs/rfcs/README.md,line=.. annotations plus a count and exits 1; on the docs(rfcs): remove committed merge-conflict markers from the registry #598 removal commit and on a clean multi-commit range it is silent with exit 0
• Markdown-scan pattern battery: labeled/bare/tab-followed/8-wide markers and ||||||| matched as designed; ======= and marker-chars-then-text correctly not matched
• Workflow file re-validated: YAML parses, scripts/check-workflow-action-pins.py green
• bash scripts/check-agents-md.sh: AGENTS.md ↔ docs indexes OK (45 links, 43 docs)
• cargo test: not run, no Rust touched

Notes for reviewers

• The two layers complement each other: the script scans all Markdown and runs locally for anyone invoking check-docs.py; the diff gate covers every file type but judges only lines a pull request adds, via git's own detector — no hand-rolled second pattern to keep in sync.
• There is no exemption, by design: an opt-out either dies at the other layer or silences whole files. A document that must quote a conflict block indents the markers one space (only column-0 markers match). The tree has zero marker lines today, so both layers start green.
• A lone ======= line is deliberately not matched by the Markdown scan (it is a legal setext heading underline); the diff gate does flag one on added lines, where git's detector owns the ambiguity.
• Scope choice: the diff gate deliberately does not scan the whole tree, so a marker landing on the base branch through a path that bypasses PRs would not turn other pull requests red; the Markdown scan still covers that case for docs.

Greptile Summary

This PR adds two complementary CI checks that reject committed merge-conflict markers and documents the new behavior.

• Scans all checked Markdown files for column-zero conflict-marker lines.
• Checks added lines across every file type using git diff --check.
• Emits inline CI annotations and documents how legitimate conflict examples should be indented.

Confidence Score: 5/5

The PR appears safe to merge, with no actionable correctness or security failures identified.

The Markdown scan and pull-request diff gate consistently enforce the intended conflict-marker policy while preserving existing required CI reporting behavior.

Important Files Changed

Filename	Overview
.github/workflows/ci.yml	Adds sufficient checkout depth and a pull-request-only diff check that annotates and rejects newly added conflict-marker lines.
scripts/check-docs.py	Adds a focused regex scan over the existing Markdown file set and reports exact locations for committed conflict markers.
docs/dev/ci.md	Accurately documents the two marker-detection layers, their scope, and the indentation convention for quoted examples.

_{Reviews (1): Last reviewed commit: "ci: reject committed merge-conflict mark..." | Re-trigger Greptile}

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