feat(emit): astrid-emit — agent-agnostic stdio→bus hook pipe - #842

Linked Issue

Closes #814

Summary

Ships the native astrid-emit binary — the core deliverable of #814. It's a tiny, agent-agnostic stdio→bus pipe that any agent's hook process shells out to: it reads stdin, wraps it in the fixed six-field sage HookEnvelope, connects to astridd as an uplink, publishes via publish-as on the given topic, writes {"continue":true} to stdout, and exits 0/1 — never 2. It carries zero agent-protocol knowledge (no hook-name map, no stdin parsing, no verdict shaping, no merge, no fail-closed). Sage is the trust anchor: it token-validates the event on the sage.v1.hook.* intake topic and republishes on canonical hook.v1.event.*; the Claude→Astrid hook-name map lives in sage, not core.

Changes

• New crates/astrid-emit (lib + bin): the pipe, plus an Emitter seam so the entire envelope contract is unit-testable without a running daemon.
• Co-installed beside astrid via an astrid-cli [[bin]] shim (src/emit.rs), mirroring the astrid-daemon/astrid-build precedent; added to the release matrix (release.yml) and README install snippet.
• astrid --emit-path discovery flag, handled before banner/config so hook-bridge installers (sage) can resolve the path on a half-configured host without side effects.
• CHANGELOG entry under [Unreleased].

Contract (pinned against the shipped sage validator)

• Six-field envelope {hook, payload, correlation_id:null, principal_id, session_id, token} — matches sage's HookEnvelope byte-for-byte.
• Topic accepted both positionally (astrid-emit <topic>, what sage writes into settings.local.json) and via --topic (issue AC); exactly one required.
• hook = topic's trailing dot-segment (matches sage's tail check); payload forwarded as a verbatim UTF-8 string (never base64, since sage forwards it opaquely to canonical subscribers).
• ASTRID_PRINCIPAL_ID / ASTRID_SESSION_ID / ASTRID_HOOK_TOKEN all required; missing-or-empty is a soft failure.

Test Plan

Automated

• cargo test --workspace passes — 12 astrid-emit unit tests (incl. the feat(cli): astrid-emit — sage-validated hook events to the bus #814 AC scenario verbatim) + 192 astrid CLI tests
• No new clippy warnings — cargo clippy -p astrid-emit --all-targets clean

Manual

• astrid --emit-path → prints the co-installed binary path, exit 0
• ASTRID_PRINCIPAL_ID=alice ASTRID_SESSION_ID=s1 ASTRID_HOOK_TOKEN=tk astrid-emit sage.v1.hook.before_tool_call <<<'{}' (and the --topic form) → {"continue":true}; soft-fail exit 1 when the daemon is down or env is missing
• malformed argv (astrid-emit --bogus) → {"continue":true} + exit 1 (never 2); --help / --version → exit 0

Notes for reviewers

• Rebased onto current main, which includes async per-capsule WASM (feat(runtime): async per-capsule WASM + instance pool + per-principal isolation (foundation for #821–826) #828) and the budget feature (feat(capsule): share the per-principal CPU fuel ledger across all capsules #821–826). The IpcPayload::RawJson → bare-object delivery that sage's serde_json::from_str(&msg.payload) relies on is verified intact post-rebase: to_guest_bytes for RawJson still returns the inner value with no type tag (astrid-types/src/ipc.rs), and the dispatcher still delivers payload.to_guest_bytes() to the guest. No conflicts, no API breakage; full workspace build + tests green.
• The sage validator side is already implemented (separate, sage repo). Once this lands and astrid-emit is on PATH, hooks flow end-to-end with zero further sage changes.
• feat(cli): astrid-emit — sage-validated hook events to the bus #814's AC text was corrected alongside this work: sage authenticates via the per-session KV token match, not a claimed-vs-envelope principal cross-check (that check never existed in the shipped validator); the binary stamps the principal via publish-as for diagnostics only.

Checklist

• Linked to an issue
• CHANGELOG.md updated under [Unreleased]