basou: Refuse a session's events.jsonl or session.yaml that is not a regular file, instead of following it

[takashi-matsuyama]

What

A session's events.jsonl and session.yaml (inside .basou/sessions/<id>/) were read through whatever stood at their names. A symlink was followed to a file outside the store, and a FIFO made basou verify, basou board measure and every reader of the session's events wait without end. basou never creates either form.

A shared reader in packages/core/src/storage/session-dir.ts now lstats the file first. A symlink, whatever it points to, or anything but a regular file (a directory, a FIFO) is refused with SessionFileNotAFileError (session id, file, kind) before anything is read; an absent file is handled as before. Every reader of the two files goes through it: replayEvents / readAllEvents, readSessionYaml and the session listing, verifyEventsChain, the chained append (so every note, decision record, task attach, approval resolution, and exec / run append and finalize), the in-place re-import, session rechain, session show, the running transition of exec and run, and the board's count of lost lines.

This is the second part of the reader checks; the first (the session's entry itself) was #356.

Behaviour

• basou verify reports such a session tampered with a new reason, events_not_a_file or yaml_not_a_file, and event_count 0; it used to follow the file (an intact log behind a symlink was verified, exit 0) or wait on a FIFO. A session.yaml that is a regular file is still read beside an events.jsonl that is not, so a newer basou's session stays unsupported and session_yaml_invalid is still reported. session rechain skips such a session with the same reasons and exits non-zero; an in-place re-import skips it as a broken prior.
• Commands that read the session by its id stop with <file> of session <id> is not a regular file; a symlink or anything else there is not followed. approval approve / reject stop for such a session.yaml (they used to go on for an unreadable one) and leave the approval pending.
• Listings leave out a session whose session.yaml is such (session_yaml_not_a_file, full id); one whose events.jsonl is such is kept, its events unreadable (events_jsonl_unreadable). import warns that such a session may be imported again; task refresh-linkage keeps a link to it.
• basou view: 404 for such a session.yaml; the session without its events for such an events.jsonl.
• @basou/sdk: listSessions / getSession leave out a session whose session.yaml is such; readEvents / streamEvents return no events for such an events.jsonl and report it to onDiagnostic as events_jsonl_unreadable, the words stats() and the renderers use (compatibility.md, "The errors may grow" item 4).
• basou board measure: integrity counts the session as verify judges it; trail, review_gaps and effort are not measured for a refused events.jsonl, and review_gaps, freshness and effort for a refused session.yaml. The five sections' method versions go from 1 to 2.
• A directory at events.jsonl, which some tests used to provoke a read failure, is now one of these cases (verify reports tampered instead of stopping); those tests make the failure with chmod instead.

Docs

• docs/spec/schemas.md section 7.5 lists the two reasons, each defined by its own file (which one is given when both apply is left open), and when session_yaml_invalid is left out.
• docs/spec/workspace.md gains "A session's files are not followed", federated mirrors included.
• docs/spec/compatibility.md gains "A layout basou never creates may come to be refused": the one way an invocation moves from 0 to a non-zero exit within 1.x, bounded by nothing changing on a layout basou creates, the refusal naming what it refuses, and the changelog listing it. This PR is such a change, as the symlink / not_a_directory reasons were.

Review

The second commit applies the findings of a convergent review and an unrequested divergent review (trigger: a published contract, the verify --json reason values and the SDK's reads), each review-only, one round, with per-finding approval: the version gate running after the layout check, session_yaml_invalid on those rows, the SDK diagnostic wording, documentation that overstated the board and the SDK, the compatibility bound, and four test gaps. Not adopted: counting the log on yaml_not_a_file rows (kept consistent with the other layout rows), and renaming the reasons (settled before the work).

Verification

• pnpm -r build, pnpm -r typecheck, pnpm lint and pnpm -r test pass (core 2788, sdk 67, cli 1555).
• New tests cover a symlink to an intact copy outside the store (a reader that followed it would succeed), a dangling symlink, a FIFO and a directory, for both files, across the readers, verify (including a newer basou's session.yaml), rechain, re-import, the appends and finalize, listings, board, view, approval, import, task linkage, exec / run and the SDK.
• Against a fixture workspace, the built CLI of main blocks in basou verify --json on a FIFO session.yaml (killed by a 15 s alarm); this branch prints the two tampered rows and exits 1.
• Mutation runs over both commits: 28 mutants of the new checks, all caught but one. The survivor replaces the in-place re-import's raw read with a following readFile, which only a swap between the verify gate and that read can reach.