Point the observer comments at the doc's current name

[monthop-gmail]

What does this change?

Four comments in overseer.ts cite observers-implementation-plan.md, but the file is checked in as docs/observers.md — so following the reference finds nothing. I hit this while reading ensureObserver() and went looking for a file that isn't there.

Retargeted all four, matching how blueprint-archive.ts already cites docs/blueprints.md:

line	now cites
397	docs/observers.md §3
2761	docs/observers.md §5 Step 5
6046	docs/observers.md §5 Step 6
6092	docs/observers.md §5 Step 3

Why is this obviously correct and trivially verifiable?

The patch is four comment lines; no code changes, so behaviour cannot move.

Both halves are checkable from the tree alone:

• docs/observers.md exists and observers-implementation-plan.md does not.
• Every cited section still exists under the new name — ## 3. Concepts & terminology, ### Step 3, ### Step 5, ### Step 6 — so each reference resolves to the passage it was always pointing at.

Checklist

• This is a small, concrete change; it is not a feature, refactor, or low-value cleanup.
• I understand that maintainers decide whether the change is obviously correct and trivially verifiable.
• I have read and followed the contribution guidelines.

🤖 Generated with Claude Code

[github-actions]

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[github-actions]

Thank you for taking the time to contribute. This pull request was automatically closed because:

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Please update the pull request to meet these automatic checks, then reopen it. Passing these checks does not guarantee acceptance; maintainers still determine whether a change is obviously correct and trivially verifiable.

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[monthop-gmail]

Superseded by #228 — I filed this one without the pull request template, so the policy checklist was missing. Sorry for the noise.