fix(cli): escape env values in the codex config.toml writer

[stephanbrez]

Problem

zenith init --agent codex writes [mcp_servers.zenith.env] into
.codex/config.toml by interpolating each value into a TOML basic string
with a raw f-string (cli.py, no escaping of ", \, or newlines):

env_lines = "\n".join(f'{k} = "{v}"' for k, v in env.items())

-c key="value" is the supported way to splice codex config through
ZENITH_*_ACP_COMMAND, so those values routinely contain double quotes:

$ zenith init --agent codex \
    --validator-acp-command 'codex-acp -c model="gpt-5.6-terra"'

emits invalid TOML — the string terminates at model=, leaving a dangling
gpt-5.6-terra"":

ZENITH_VALIDATOR_ACP_COMMAND = "codex-acp -c model="gpt-5.6-terra""

Codex then fails to parse config.toml or drops the whole managed block, and
the role silently falls back to defaults. Because the line loops over the
entire env dict, the same hole affects every role's ACP command and any
forwarded allowlisted value (ANTHROPIC_MODEL, ZAI_API_KEY, base URLs)
that contains a quote or backslash.

Fix

Serialize with json.dumps. TOML basic strings share JSON's escape syntax,
and json.dumps emits only escapes TOML accepts (\", \\, \n, \t,
\uXXXX; never \/ by default). This mirrors the
args = {json.dumps(server_args)} line directly below it, and fixes every
role at once since the loop spans the whole env dict. Env var names are valid
TOML bare keys, so keys are unchanged.

Only the codex_config branch was affected — the mcp_json branch hands the
same env dict to json.dumps and has always been correct.

Why this surfaced now

Related: #31. Before that change, a custom -c model="..." inside
ZENITH_*_ACP_COMMAND was silently discarded by the npm codex-acp adapter,
so there was little reason to pass a quoted -c value to
--worker-acp-command. #31 makes those overrides functional by routing them
through CODEX_CONFIG, which turns this latent escaping hole into reachable
config corruption.

This PR has no code dependency on #31. It applies cleanly to main as-is,
does not touch acp_runner.py, and can merge before, after, or independently
of that PR. (_augment_acp_command has always appended
-c sandbox_mode="danger-full-access", but that happens at dispatch and never
reaches config.toml; only user-supplied commands are written by
zenith init.)

Testing

New test_codex_init_escapes_quoted_acp_commands drives zenith init --agent codex with a distinct quoted command for worker and validator, asserts
config.toml parses via tomllib, and asserts each value round-trips
byte-identical. It raises TOMLDecodeError without the fix.

Existing codex config tests are unaffected — json.dumps on quote-free values
produces byte-identical output to the current code.

• uv run pytest -q — 213 passed, 7 skipped
• uv run ruff check . — passed
• uv run mypy src — passed
• Verified end-to-end: before the fix tomllib.loads on the generated
  config.toml raises; after, all commands parse with quotes intact.

🤖 Generated with Claude Code