fix(observability): guard against nil tag before sort

[TimeToBuildBob]

Summary

• site.tags["observability"] returns nil when no posts carry the tag
• | sort: "date" on nil throws Cannot sort a null object. in Liquid
• Fix: nil-check before sort, fall back to "" | split: "" (empty array)

Impact

This was breaking the weekly gptme digest publish step (bob-weekly-gptme-digest.service) — the pre-commit Jekyll build failed, blocking the website PR from being created.

Test plan

• Jekyll build passes with the nil guard in place (pre-commit passed locally)
• Site builds without error when no posts are tagged observability

[TimeToBuildBob]

🤖 AI code review

The PR modifies observability.pug to guard against a nil value for site.tags["observability"] before sorting. It assigns the raw tag array to obs_posts_raw, then conditionally sorts and reverses it if non-nil, otherwise assigns an empty array via "" | split: "". This prevents a Liquid 'Cannot sort a null object' error when no posts carry the observability tag.

Safe to merge — no P0/P1 findings

Confidence 5/5

✅ No thread-worthy findings. Advisory notes follow; they are retained without opening review threads.

1 advisory finding (summary-only, not scored)

These P2 guard, heuristic, trade-off, or documentation claims are retained for judgment without opening review threads.

⚠️ P2 medium — observability.pug

This is a fix(...) PR but no test files are included in the diff. Erik's feedback: 'where is the repro & fixes they are supposed to catch' (gptme#3441), 'that measurement should come with a regression test' (gptme#3446). Add a test that would have caught this bug. (Advisory: Erik merged all such PRs but consistently requested tests.)

Add a test file that reproduces the bug before the fix and passes after it.

_{How this was verified: static preflight: fix-commit + touched-files scan (rule 7)}

Files changed (1) — the diff as I read it

• observability.pug — Adds a nil guard around the sort of site.tags["observability"] and falls back to an empty array when the tag is absent.

_{Reviewed d77d9dcc65ef · openrouter/deepseek/deepseek-v4-flash-0731 · llm engine · 4s · about this reviewer}

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_{@TimeToBuildBob review (own line) — fresh review · @TimeToBuildBob fix — a worker acts on the findings. Once per comment; 👀 = received.}