docs(community): showcase two external Opik projects + allow external proof_url

[LeoRoccoBreedt]

What & why

Showcases two external, community-built projects that use Opik, as listed entries in community/:

• Building a Coding Agent from Scratch (course) — Paul Iusztin / Decoding AI. Opik is used for tracing + evals (Lesson 7).
• Observable Job Agent — Shirin Khosravi Jam / jamwithai. Observability-first CV→job-matching agent instrumented with Opik from run one.

Both link out to the authors' own repos (hosted: false); no external code is vendored in.

Since these are maintainer-added showcases (not author self-submissions), we don't hold the authors' Opik screenshots. So this PR also relaxes the community proof rule: an entry can now prove Opik usage with an http(s) proof_url in meta.yaml (pointing at the author's own trace screenshot / setup doc) as an alternative to a committed opik-proof.png. The committed-PNG path is unchanged for normal author submissions.

Changes

• community/_ci/entry_rules.py — validate_proof accepts a committed+referenced opik-proof.png or an http(s):// proof_url; signature unchanged, PNG path behavior preserved.
• community/_ci/tests/test_entry_rules.py — new tests for the proof_url accept path, the non-http reject, the scheme-less (httpfoo) reject, and no-proof reject. Suite: 51 passing.
• community/templates/entry-template/meta.yaml + community/CONTRIBUTING.md — document the optional proof_url.
• Two new listed entries (paul_iusztin_coding_agent_course/, jamwithai_observable_job_agent/), each meta.yaml + README.md.
• community/README.md — regenerated by build_index.py (two-row index).

Each proof_url points at a real, public page in the author's repo showing Opik usage (a traces screenshot and an Opik setup/tracing doc, respectively) — no fabricated proof.

Checklist

• Change confined to community/ (its lighter-weight contract, not the root example contract)
• Folder names are lowercase_with_underscores and pass community/_ci/check_entry.py
• Both entries have all four required README sections and a valid meta.yaml; author is the owning GitHub org so github.com/<author> resolves
• Proof of Opik usage present (real external proof_url for each)
• community/README.md regenerated via build_index.py (--check clean)
• python -m pytest -q in community/_ci — 51 passing
• No credentials or .env files committed

🤖 Generated with Claude Code