docs: add CONTRIBUTING.md

[vernonstinebaker]

Adds the contribution flow: build prerequisites, commit gates (zig build test, zig fmt, ui build), the 4-target CI matrix including the e2e suite, the regression-test mandate, and a lifecycle-hygiene rule for instance process management (distilled from #86). Complements the existing AGENTS.md. Docs-only.

[DonPrus]

Checked the guide against AGENTS.md, README.md, build.zig, ui/package.json, the actual CI inputs and #86. The four-target matrix and native-Linux E2E description match the workflow. The lifecycle requirement is appropriate for the existing supervisor/CLI boundary, and the guide adds a contribution flow rather than duplicating the architecture documentation. It merges cleanly with current main.

One non-blocking improvement: it would be great to use bash tests/test_backend.sh as the backend gate (or list both commands from it). CI and README currently run both zig build test and zig build test-integration with -Dembed-ui=false -Dbuild-ui=false; the new checklist only mentions unit tests. Using the existing script keeps the instructions consistent, includes the real-process HTTP integration suite, and avoids an unnecessary UI build for backend-only checks.

No blocking findings in this documentation-only PR.

[vernonstinebaker]

Addressed in d76930e — step 4 now uses bash tests/test_backend.sh, the script CI actually invokes (ci.yml:30 test_command), and names both commands it wraps (zig build test + zig build test-integration, both with -Dembed-ui=false -Dbuild-ui=false).

That also fixes a second inaccuracy in the old line: it said zig build test --summary all with no -D flags and no integration suite, while README:211-212 and CI both use the flags and run both steps.

Left the "4-target CI matrix" wording as-is — verified it's correct here: targets_json in ci.yml has exactly those 4 entries and all 4 are required checks (unlike nullwatch, where the shared default is 3).

Docs-only, +20/-0 on one new file. CI running; approval retained.

[DonPrus]

Rechecked d76930e against tests/test_backend.sh and .github/workflows/ci.yml. The contributor gate now matches both backend unit and integration checks, including the UI build flags. The four-target matrix remains accurate. The earlier nit is resolved; approval stands.