fix(kernel): enumerate run-loop capsule tools via the describe fan-out (#1198)

[jvsteiner]

Linked Issue

Closes #1198.

Summary

A run-loop capsule's tools could remain invisible. Load-time interceptor discovery is unavailable for pool-less run-loop capsules, but persisted or injected tools: [] metadata suppressed the existing describe fan-out that should query the capsule's live responder.

Changes

• Add fan-out-aware discovery status while preserving the existing public describe_loaded_capsule signature.
• Leave tools absent when the interceptor path is unavailable so tool.v1.request.describe fan-out runs.
• Treat tools as live-owned metadata and remove legacy or stale tool surfaces before each probe while preserving unrelated opaque metadata.
• Cover every result-mapping branch plus legacy object metadata, absent metadata, and non-object metadata.

Verification

• cargo fmt --all -- --check
• Tool-discovery tests: 16 passed
• cargo test -p astrid-kernel --lib -- --quiet: 304 passed
• cargo clippy -p astrid-capsule -p astrid-kernel --all-features -- -D warnings
• git diff --cached --check

AI / Tool Assistance

Assisted-by: Anthropic Claude: Opus 4.8
Assisted-by: OpenAI Codex: GPT-5

Claude assisted Jamie with the original implementation. Codex independently reviewed the live discovery and persisted-metadata behavior, fixed the stale metadata suppression path, added regressions, reconciled the patch onto current main, and validated the affected crates. I reviewed the final combined patch and created the GPG-signed, DCO-compliant commit. Jamie and Claude remain credited as co-authors.

Checklist

• Linked to an issue
• CHANGELOG.md updated
• I understand every change in this PR and can explain its design, risks, and validation.
• I reviewed and tested any meaningful tool-generated output included in this PR.
• Every non-bot, non-merge commit has a matching Signed-off-by trailer.

[jvsteiner]

@joshuajbouw ready for review — pool-less run-loop capsules now leave tools absent so the describe fan-out fires and their tools enumerate (#1198).

[joshuajbouw]

Review outcome: useful fix, but not mergeable as posted. The public Rust API compatibility job fails because describe_loaded_capsule changes from Result<Vec<ToolDescriptor>> to Result<Option<Vec<ToolDescriptor>>>. Please preserve the existing public signature with an additive/internal API, or explicitly handle this as an intentional breaking change under the project’s release policy. The PR template and changelog also need updating.