fix(panes): /tui works in panes with workspace instructions

Claude refused `/tui` ("Cannot switch renderers in this session — it has
restrictions a restart can't carry over") in every pane started with
--append-system-prompt-file, i.e. any workspace with instructions (member
panes usually have the owner's). The instructions are now written as the only
CLAUDE.md of a folder of their own (<data dir>/workspace-prompts/<id>/, a
member's prompt-<id>/ in the handoff dir via write-as), passed as one more
--add-dir with CLAUDE_CODE_ADDITIONAL_DIRECTORIES_CLAUDE_MD=1. Verified with
Claude Code 2.1.301: the file is listed in /memory and /tui switches. Codex
is unchanged (its pointer line names the same file).

docs/API.md

@@ -1598,7 +1598,7 @@ What a workspace's home page edits (`server/lib/lane-sources.js`, `server/routes
 - **Memory** — read-only: the folder's `CLAUDE.md`, `.claude/CLAUDE.md`, `CLAUDE.local.md`, `AGENTS.md`, and for a Claude workspace `<CLAUDE_HOME>/projects/<encoded cwd>/memory/*.md` — `{files: [{label, path, size, mtime, truncated, content}]}`, each at most 256 KB of content.
 - Every write is same-origin only (`403 EBADORIGIN`); an unknown workspace is `404 ENOLANE`, an unknown source `404 ENOSOURCE`.
 
-At spawn (`spawnRun`, both `POST /api/lanes/:id/start` and `POST /api/run`) the pane gets, from these rows: Claude — `--add-dir <folders…> [<context dir>]` first, then `--append-system-prompt-file=<data dir>/workspace-prompts/<lane id>.md`, a file rewritten at every spawn holding the workspace owner's own instructions (`PUT /api/me/instructions`) + the workspace's instructions + the context file list + the extra folders; Codex — one `--add-dir <dir>` per folder (and the context dir) and `-c developer_instructions=<same text as a TOML string>` (above 6 KB as the escaped TOML string, one line pointing at that file instead; nothing when that file could not be written). Any other tool gets none of it (`supported: false`). A folder or file that vanished is skipped.
+At spawn (`spawnRun`, both `POST /api/lanes/:id/start` and `POST /api/run`) the pane gets, from these rows: Claude — `--add-dir <folders…> [<context dir>]` first, then that workspace's instructions folder `<data dir>/workspace-prompts/<lane id>/` (a member's: `prompt-<lane id>/` in their handoff dir) as one more `--add-dir`, with `CLAUDE_CODE_ADDITIONAL_DIRECTORIES_CLAUDE_MD=1` in the pane's env so Claude reads the `CLAUDE.md` there as memory — never `--append-system-prompt-file`: Claude cannot carry a custom system prompt over a restart and refused `/tui` in such a pane. That `CLAUDE.md` is rewritten at every spawn and holds the workspace owner's own instructions (`PUT /api/me/instructions`) + the workspace's instructions + the context file list + the extra folders; Codex — one `--add-dir <dir>` per folder (and the context dir) and `-c developer_instructions=<same text as a TOML string>` (above 6 KB as the escaped TOML string, one line pointing at that file instead; nothing when that file could not be written). Any other tool gets none of it (`supported: false`). A folder or file that vanished is skipped.
 
 #### Projects (workspaces grouped by repository)