feat(vigil): Janet plugin bridge and hooks (slice 4 of 5)

[allen-munsch]

Summary

Slice 4 of the vigil feature stack: the Janet plugin bridge and lifecycle hooks. This is the piece that lets Janet plugins interact with the running vigil-keeper at runtime.

• Exposes vigil/live?, vigil/emit, vigil/list, vigil/get, and vigil/set-state to Janet plugins via a process-global OnceLock bridge in the plugin worker.
• Registers the three vigil lifecycle hooks (on-vigil-event, on-vigil-reap, on-vigil-observance) in the loader so harness/register-hook accepts them.
• Installs the keeper's plugin event channel into the bridge at startup so (vigil/emit name data) reaches the keeper router across the worker/runtime thread boundary.

Scope

• src/plugin/worker.rs — the vigil_bridge module (statics, install functions, C functions, HARNESS_VIGIL_INIT prelude) plus unit tests.
• src/plugin/loader.rs — three hook names appended to HOOK_NAMES.
• src/main.rs — installs vigil_plugin_tx + vigil names after the keeper is created (behind #[cfg(feature = "plugin")], nested inside the existing #[cfg(feature = "vigil")] region).

Stack

• feat(vigil): vigil persistence and dirge vigil CLI (slice 1 of 5) #810 — slice 1: persistence + dirge vigil CLI
• feat(vigil): core runtime and --vigil-once headless driver (slice 2 of 5) #811 — slice 2: core runtime + --vigil-once
• feat(vigil): interactive TUI wiring (slice 3 of 5) #817 — slice 3: interactive TUI wiring
• this PR — slice 4: Janet plugin bridge + hooks
• slice 5 (not yet opened): docs + e2e fixtures

Testing

Local reproduction of the CI matrix before opening:

• cargo build --bin dirge for vigil, plugin, windows-default, no-plugin, and default feature sets — all clean.
• cargo clippy --bin dirge for all-features, windows-default, no-plugin, and default — zero warnings.
• cargo fmt --all --check — clean.
• cargo test --bin dirge --all-features -- --skip sandbox::microvm --skip ui::relay_tests — 5925 passed, 0 failed.

Known environment-dependent gaps (not caused by this change, and not exercised in CI):

• The 13 sandbox::microvm OCI-image tests fail locally (podman/buildah environment).
• The ui::relay_tests PTY integration tests hang locally under load; they are gated on sandbox-microvm and unrelated to the plugin bridge.

Self-review notes

• vigil_bridge::json_to_janet duplicates the existing free json_to_janet (which goes through a depth-limited json_to_janet_depth). The bridge copy is recursive without a depth cap, so a pathologically nested value set via vigil/set-state could recurse deeply. It's bounded by what a plugin sets, but reusing the depth-limited free function would be cleaner.
• The module-level doc comment still says "Thread-local mpsc sender" while the code (and the comment directly below it) correctly use a process-global OnceLock; that first line is stale and should be corrected.