feat(sessions): add persistent display names

[alx-xo]

Goal

Kit generates session titles from the earliest useful user message, which is not always memorable once a workspace has many durable sessions. This adds an optional persistent display name while preserving the generated title as the fallback.

What changed

CLI

• kit sessions shows the custom name when set, otherwise the generated title.
• kit sessions rename <session-id> "OAuth token bug" sets or replaces a name.
• kit sessions rename <session-id> --clear restores the generated title.
• --root works on both listing and rename commands.

TUI

• /sessions keeps the existing selection and resume flow.
• R opens an inline rename field; submitting an empty value asks before clearing.
• Saving keeps the picker open, preserves the selected row, and refreshes its name.
• The picker footer documents select, resume, rename, and cancel controls.

Persistence and compatibility

• Names live in an atomic workspace-scoped sidecar, separate from the append-only transcript and mutation lock.
• Renaming never changes the immutable session ID, transcript, generated title, or preview.
• Missing or malformed optional metadata falls back to the generated title without hiding the session.
• Names are trimmed, limited to 100 Unicode characters, and reject terminal/bidi controls.
• CLI, TUI, and ACP session listings use the same custom-name-first fallback behavior.

Verification

• Storage tests cover set, replace, clear, malformed metadata, active sessions, workspace isolation, and concurrent writes.
• CLI integration tests exercise listing and both supported --root positions.
• TUI tests cover editing, clear confirmation, selection preservation, paste handling, and rendering.
• cargo +1.94.0 fmt --all -- --check
• cargo +1.94.0 clippy --all-targets --all-features -- -D warnings
• cargo +1.94.0 test --all-targets --all-features

Closes #66