Record recurring errors on the tracked issue instead of filing again

[FlyLikeAPenguin]

What changed?

An error whose root cause is already tracked no longer opens a second ticket. The investigation now returns is_duplicate + duplicate_of_url, and the pipeline posts a recurrence note on that issue instead of filing a new one — with the fingerprint, occurrence count, Datadog link, RCA doc link, and root cause.

Duplicates also skip the fix stage (the tracked issue owns that fix) and stay quiet on Slack.

Why?

A real case: an investigation identified an error as a duplicate of an existing epic, named the tracked issues and the open PR that fixes it, and explicitly recommended not attempting a fix. A brand new ticket was filed anyway.

Nothing in the pipeline could stop it. The investigation returned only is_actionable and is_fixable. The dedup instruction told the agent to set is_fixable=false — which gates the fix stage — while the tracker stage is gated on is_actionable alone. A duplicate of a real, unfixed bug is still actionable, so the ticket was always filed and the "already tracked" reasoning survived only as prose in the issue body.

is_duplicate is therefore deliberately independent of is_actionable; overloading is_actionable would have meant lying about whether the error matters. A duplicate claim with no issue URL is downgraded to a normal filing, since there would be nowhere to record the recurrence.

Validation

• pytest — 64 passed, including 15 new tests in tests/test_duplicates.py covering the output contract, DB round-trip and migration, recurrence-note content, and pipeline routing (tracked → comment only, untracked → issue only, non-actionable → neither)
• ruff check .
• Documentation updated where behavior changed (CONTEXT.md, CHANGELOG.md)
• No secrets, production logs, or private source code included

Risk and operations

• Migration: two additive columns on investigations (is_duplicate, duplicate_of_url), applied by _migrate() on the next run. Verified against a copy of a live database; pre-existing rows default to 0/''.
• New tracker capability: comment() added to the IssueTracker protocol and both backends. Any third-party tracker implementation needs it. GitHub's attach_pr now routes through it and picks up the --repo flag it was previously missing.
• Behaviour change worth knowing: the agent's notion_page_url/linear_issue_url output keys are no longer read. Neither was in the documented output contract and both were overwritten by the pipeline. That overwrite also broke the Slack already-reported check — it compared the just-created URL against earlier runs, so it never fired. Duplicates are now suppressed directly instead.
• Cost: unchanged. Duplicates are still investigated (that is how they are recognised); only the filing changes.