fix(validate + session): the reserved skill:: namespace is writable, and only the opener may discard a session

[jrosskopf]

Two fixes, both surfaced by testing Heron against the real write path rather than through fixtures. They ship together because the second was found while verifying the first.

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1. [[skill::<id>]] was resolvable but not writable (#424)

The documented way to reference a skill definition page — read.rs calls the namespace reserved, resolve constrains it on page_type = 'skill' (#212).

The validator did not know. It treated skill as a skill id, looked for a skill page called skill — which never exists — and refused every page using the form:

unknown_skill: wikilink references unknown skill `skill`

Two places were wrong, and fixing either alone changes the error rather than the outcome: the collection pass asked for the skill named skill instead of the link's id segment, and the existence check looked up the same wrong key.

How it surfaced: Heron's workshop formats reference a shared facilitation procedure exactly this way. Its Rust fixtures write straight to the store via FixtureBuilder, so validation never ran on them — the first thing to author a format the way a tenant would, an app test over the real write path, was refused immediately.

Worth noting for this repo too: FixtureBuilder bypassing validate means downstream suites can be green over content the gateway would reject.

Control: a reserved-namespace link to a skill that does not exist is still an error. Without it, the fix for a false positive becomes a dangling-link hole.

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2. Anyone holding a session id could discard someone else's session (#425)

close_session {commit: true} re-checks the write ACL. {commit: false} checked nothing, deliberately — a caller whose grant was revoked mid-session must still be able to abandon their own work rather than wedge the page until the idle TTL.

That exemption could not tell the opener from anyone holding the session id:

bob (team-blue) → close_session {commit:false} on a team-red session → ok: true
the owner can then reopen the page — proving their session was destroyed

No content is read, so it is not a disclosure. It is a cross-engagement denial of service, and in a room the symptom is the facilitator's session dying mid-sentence.

Sessions now record the verified subject that opened them: admin passes, the opener passes, anyone else must be able to write the page — the same bar open_session sets. An unknown session id is treated as nothing to protect, so it cannot be distinguished from a forbidden one by probing.

Control: the opener can still discard their own session. A fix that simply required the write ACL would strand exactly the caller the exemption was written for.

Found by an AI review of Heron's P5 work and reproduced before fixing.

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Verification

1356 tests pass, fmt and clippy clean.

🤖 Generated with Claude Code