docs: sync documentation with recent main merges

[aheritier]

Covers doc gaps from PRs merged to main in the past 36 hours.

Source PRs covered:

• feat(tui): add /custom command to customize layout #3509 feat(tui): add /custom command to customize layout — added /custom layout dialog and section, but the slash commands reference table was missing the entry. Added it.
• feat(board): show intermediate startup statuses on cards #3516 feat(board): show intermediate startup statuses on cards — introduced starting → loading → attaching card startup progression with no docs update. Documented the three phases in docs/features/board/index.md.
• fix(cli): resolve session DB default against the data dir #3503 fix(cli): resolve session DB default against the data dir — updated cli/index.md and acp/index.md but left docs/features/a2a/index.md missing the --session-db flag in its Flags table. Added the row.

Reviewed and already up to date:

#3517, #3513, #3514 (board: shell shortcut, project editing, env var rename — board docs already reflect these), #3504 (auto theme — TUI and CLI docs already updated), #3505 (theme watcher — no doc change needed per PR), #3507 (dep bumps — no docs), #3506, #3497 (changelogs), #3518, #3515, #3510, #3508, #3491 (bug fixes, internal — no user-facing change), #3499, #3498, #3496, #3489, #3502, #3494 (each updated their own docs).