fix(codex): denormalize Edit/Write to apply_patch on export

[Drix10]

Issue: #59

What changed and why?

to_common already normalizes Codex's native tools into canonical ones
(exec_command/shell → Bash, apply_patch → Edit/Write), but
from_common had no reverse mapping: it emitted the canonical names back as
function_call lines, which the Codex runtime cannot validate, breaking
txcript continue <id> --with codex.

Now from_common replays native shapes:

• Before: {"type":"function_call","name":"Bash","arguments":"{...}"}
  After: {"type":"function_call","name":"exec_command","arguments":"{\"cmd\":\"ls\"}"}
• Before: {"type":"function_call","name":"Edit",...}
  After: {"type":"custom_tool_call","name":"apply_patch","input":"*** Begin Patch\n*** Update File: src/main.rs\n@@\n-old\n+new\n*** End Patch"}
• Write → *** Add File envelope; multi-file Raw ApplyPatch passes its
  envelope through unwrapped. Patch results emit custom_tool_call_output
  (with exit_code carrying the error bit) so they pair on replay.
• Bash extras with no native slot (timeout, description, background) are
  dropped — same lossiness as the inbound normalizer, noted in a comment.
  replace_all degrades to a single application (no envelope equivalent).

Tests: extended the codex integration module per tests/README.md —
exec_command emission asserts the parsed cmd field and that no "Bash"
name leaks; new test proves the Edit envelope round-trips through
to_common losslessly, including is_error. Reproduced first: the three
from_common_denormalizes_* tests fail on main, all 12 pass with the fix.

Checks: cargo test -p txcript --no-default-features --test integration codex
(12 passed), cargo fmt --check and cargo clippy -p txcript --all-targets
(zero warnings) — run under the GNU toolchain, as my machine has no MSVC
linker; CI's MSVC run will confirm.