feat(build): package capsule-declared Skill assets

[joshuajbouw]

Linked Issue

Closes #1281

Summary

Makes [[skill]] a real distributable capsule asset for Rust-built capsules and preserves typed filesystem absence at the guest boundary. This lets an agent runtime discover workflows from its installed capsule introspection view without per-principal lifecycle-hook copies.

Changes

• parse synthesized Rust capsule manifests for [[skill]].file declarations
• package each declared Skill at its exact relative archive path
• reject absolute paths, traversal, missing/non-file assets, and symlinks escaping the capsule source tree
• deduplicate declared paths deterministically
• document and test declarative assets flowing through the existing principal introspection mirror
• map native VFS not-found and permission-denied I/O kinds to the existing typed guest error codes
• add packaging, traversal, out-of-tree symlink, mirror, and error-mapping regressions

Verification

• cargo test -p astrid-build — 20 passed
• cargo test -p astrid-capsule-install principal_introspection — 5 passed
• cargo test -p astrid-capsule error_mapping_tests — 2 passed
• cargo clippy -p astrid-build -p astrid-capsule-install -p astrid-capsule --all-features -- -D warnings
• cargo fmt --all -- --check
• built AOS Forge, System, and Skills capsules with this builder; the resulting archives contained all three declared SKILL.md assets and passed AOS's archive-level validator

No public API or wire shape changes. No release, version bump, channel promotion, or crates.io publication was triggered.

Checklist

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

[Copilot]

Pull request overview

Packages capsule-declared skill assets into Rust-built .capsule archives and ensures guest-visible filesystem errors preserve typed semantics (not-found / permission-denied) so runtimes can safely distinguish optional-absent inputs from real I/O failures. This aligns Rust capsule builds with the existing principal-scoped introspection mirror so skills can be discovered from installed capsule metadata without per-principal copy hooks.

Changes:

• Parse synthesized/merged Capsule.toml for [[skill]].file entries and include those files in the capsule archive at their declared relative paths (with traversal/absolute/out-of-tree validation and deterministic deduping).
• Preserve typed guest error codes for native std::io::ErrorKind::{NotFound, PermissionDenied} when wrapped by astrid_vfs::VfsError::Io.
• Add/extend regression tests for archive packaging, introspection mirroring of declarative assets, and error-code mapping; document the behavior in CHANGELOG.md.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
crates/astrid-capsule/src/engine/wasm/host/fs/mod.rs	Map VfsError::Io not-found/permission-denied into typed guest ErrorCodes and add regression tests.
crates/astrid-capsule-install/src/principal_introspection.rs	Clarify security/docs to include declarative public assets in the mirror and extend mirroring tests to cover a skill asset.
crates/astrid-build/src/rust.rs	Resolve and validate [[skill]].file assets and pass them into archive packing; add packaging/traversal/symlink-escape tests.
CHANGELOG.md	Document the new skill-asset packaging behavior and typed error mapping under [Unreleased].

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