fix(config): honor $PORT directly (PR #2 follow-up)

[aaronbrethorst]

Summary

• Teaches internal/config to honor $PORT so deployments on Render/Heroku/Fly/Cloud Run don't need a hardcoded HOOKS_LISTEN_ADDR=:10000 envVar. New precedence: HOOKS_LISTEN_ADDR > yaml listen_addr > :$PORT > :8080.
• Drops HOOKS_LISTEN_ADDR=:10000 from render.yaml; the platform-injected $PORT now drives the bind address automatically.
• A set-but-unparseable PORT (typo, out-of-range, 0) errors at Parse time rather than silently falling back to :8080 — silent demotion would mask "service unreachable" failures behind a default the PaaS load balancer can't reach.

Resolves the follow-up flagged in PR #2: "Render PORT contract... The cleaner fix is to teach internal/config to honor $PORT directly."

Test plan

• go test -race ./... — all packages pass
• make lint — 0 issues
• New table-driven TestListenAddrPrecedence covers 8 cases: PORT honored, yaml beats PORT, HOOKS_LISTEN_ADDR beats both, unset → default, and three error paths (non-numeric, out-of-range 70000, port 0)
• Exercised the binary end-to-end:
  • PORT=12345 → server logs addr=:12345 and /readyz returns 200
  • PORT=garbage → refuses to start with config: PORT="garbage" is not a valid TCP port (1-65535)
  • PORT=70000 → same error path
  • HOOKS_LISTEN_ADDR=:23456 PORT=12345 → binds :23456 (override wins)

Review notes

• An invalid PORT errors regardless of whether HOOKS_LISTEN_ADDR is also set — i.e. the env override does not "rescue" a typo'd PORT. This is intentional (loud-fail principle) and the operator can fix or unset PORT if they hit it. Happy to make HOOKS_LISTEN_ADDR short-circuit the validation if reviewers prefer that contract.
• PORT=0 is rejected by the 1 <= n <= 65535 guard. Some Go programs use :0 to mean "OS picks an ephemeral port" via net.Listen; PaaS providers never inject 0, so locking it out is fine for this server but worth flagging.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: a9cd5952-c0e8-44c5-ac2c-120a72441818

📥 Commits

Reviewing files that changed from the base of the PR and between 08cac0a and 24d5fad.

📒 Files selected for processing (5)

• CLAUDE.md
• docs/quickstart.md
• internal/config/config.go
• internal/config/config_test.go
• render.yaml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fixes

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud