fix(interactive): hand tmux a script path, never the system prompt (#96)

[Lexus2016]

Closes #96.

Root cause

A tmux new-session command travels over tmux's imsg socket, which caps the whole message at ~16 KB. Reproduced on tmux 3.7c (macOS): 16 300 B starts, 16 340 B → failed to send command, 20 000 B → command too long. runInteractiveSingle() put the entire --append-system-prompt (studio prompt + skills + AGENTS.md, up to 64 KB) inside that command, and spawned tmux with stdio:'ignore', so the only thing on screen was the generic "failed to start tmux session".

Fix (claude-interactive.js)

• tmuxLaunchCommand() writes the full invocation into a content-addressed script inside a per-process mkdtemp (0700) directory and hands tmux sh <path> — ~60 bytes regardless of prompt size. The script execs, so the pane process is still claude.
• Script rather than "$(cat file)" (the suggestion in the issue): tmux runs the command through default-shell, the user's login shell — $( ) is a syntax error under csh/tcsh and strips trailing newlines elsewhere. sh <path> is plain words in every shell.
• Private dir, not a predictable /tmp name: this file is executed, so a pre-created file by another local user would be run instead of ours.
• Unwritable tmpdir → falls back to the pre-Bug: Interactive engine fails to start for any project with an AGENTS.md larger than ~16 KB (tmux "command too long") #96 inline command (works below 16 KB as before).
• tmux stderr is now piped and appended to the error frame, so command too long reaches the user.
• buildInteractiveCommand() extracted (pure) so the argv shape is unit-testable.
• agents-md.js: MAX_BYTES comment corrected — it is an argv budget (macOS ARG_MAX is 1 MB, not 256 KB); the tmux ceiling no longer applies.

Test

test/engine-spawn-cmd.test.js (wired into npm test): 15 pure checks + an end-to-end run through a real tmux on a private socket with a fake claude that dumps its argv — asserts the 40 KB prompt (with non-ASCII) arrives byte-identical. The pre-fix inline form runs as an informational control and prints command too long.

$ node test/engine-spawn-cmd.test.js
  info inline command (40224 B) → command too long
  ok   tmux accepted the launch command
  ok   child received the 40 KB prompt byte-identical
21 passed, 0 failed

$ npm test   → EXIT=0 (79 files, 59 plain-node suites "0 failed")

Cost / risk

• One small script file per distinct config per process, in a 0700 tmp dir; not cleaned up (same lifecycle as the existing ccs-mcp-*.json). Content-addressed, so a respawn with the same config reuses it.
• The pane's start command is now sh /tmp/ccs-spawn-…/spawn-<hash>.sh instead of the readable claude … line; ps still shows the exec'd claude argv.
• Does not touch the API engine or SSH paths — they never went through tmux.

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