fix(tui): write the command history owner-only

[clawedassistant26]

The bug

The pit saves every line typed at the mosh prompt to ~/.moshcode_history, and saveHistory() passed no mode, so the file is created 0644 under a normal umask. Any other account on the machine can read it.

That matters because the documented TUI flows put secrets on the command line:

• /mcp install <url> -H "Authorization: Bearer <token>"
• /secrets … (logicsrc credentials), /coinpay …, /ugig …
• the ! shell escape, e.g. !export STRIPE_KEY=sk_live_…

src/auth.mjs already writes credentials.json with mode: 0o600, so the standard for secret-bearing files in this repo is owner-only. The history file just missed it.

Repro (executed on unmodified main, real TTY)

$ T=$(mktemp -d)
$ printf '/pwd\n/mcp install https://mcp.example.com/sse -H "Authorization: Bearer sk-live-SUPERSECRET"\n/quit\n' \
    | HOME=$T script -qec "node bin/moshcode.mjs" /dev/null
$ stat -c %a $T/.moshcode_history
644
$ cat $T/.moshcode_history
/quit
/mcp install https://mcp.example.com/sse -H "Authorization: Bearer sk-live-SUPERSECRET"
/pwd

The fix

src/tui.mjs only:

• saveHistory() writes with { mode: 0o600 }.
• It then chmodSync(…, 0o600). This is the part that matters for existing users: Node applies mode only when the file is created, so every history file already on disk would otherwise keep its 0644 bits forever. Tightening on each save fixes those in place.
• Still fully best-effort inside the existing try/catch — history is a convenience and must never be fatal. On Windows chmod is a no-op for these bits and does not throw.

Tests

Two added to test/tui.test.mjs, both driving the real CLI in a child process with HOME pointed at a temp dir:

1. a fresh history file is 0600;
2. a pre-existing 0644 history file is tightened to 0600 and its contents survive.

Both skip on win32 (POSIX permission bits).

Stash-verified with git stash push -- src/: both FAIL unpatched (9 pass / 2 fail), 11/11 with the fix.
Full suite node --test: 166 tests / 163 pass / 0 fail / 3 skipped (main baseline measured this run: 164 / 161 / 0 fail / 3 skipped).