Add user.prompt_submit and stop project hooks to code-rs

[voarsh2]

Summary

This PR extends code-rs project hooks with two new lifecycle events:

• user.prompt_submit
• stop

The goal is to bring code-rs closer to upstream Codex hook behavior for
prompt-time context injection and end-of-turn continuation, while staying
inside the existing project-hook architecture.

What changed

New project hook events

Added support for:

• user.prompt_submit
• stop

For compatibility with upstream naming, UserPromptSubmit and Stop are
accepted as config aliases.

Prompt-submit semantics

user.prompt_submit now supports minimal upstream-inspired behavior:

• exit 0: continue normally
• exit 2 with non-empty stderr: block the prompt
• non-empty stdout: inject developer context into model input before the turn starts

This enables integrations that need to add context at prompt time.

Stop semantics

stop now supports end-of-turn continuation:

• exit 0: allow the turn to finish normally
• exit 2 with non-empty stderr: treat stderr as a continuation prompt
• multiple blocking stop hooks are joined with blank lines
• stop-driven continuation is limited to one extra turn to avoid recursive loops

Hook payload enrichment

To support integrations that need richer lifecycle metadata, hook payloads now include:

• session_id
• turn_id where applicable
• transcript_path
• model

This applies to session.start, user.prompt_submit, and stop.

TUI fix

Also fixes a debug-build TUI assertion where a background event inserted during
prelude/startup could be tagged incorrectly and panic.

Validation

Focused tests

Passed:

• cargo test -p code-core --test tool_hooks -- --nocapture
• cargo test -p code-core project_hook_event_deserializes_upstream_aliases -- --nocapture

Coverage includes:

• prompt-submit firing
• blocked prompt-submit skipping model submission
• prompt context injection reaching model input
• stop firing
• joined stop continuation prompts
• loop guard behavior
• session/stop metadata presence
• no duplicate stop continuation prompt in follow-up requests

Build

Passed:

• cargo build --bin code --bin code-tui --bin code-exec

Smoke validation

A built-binary smoke harness was used during development to verify:

• user.prompt_submit fires
• stop fires
• injected prompt context reaches model input
• stop continuation prompt reaches model input once

Non-goals

This PR does not add:

• global hooks
• full upstream Codex hook-runtime parity
• tolerant parsing for older binaries that do not know these new event names

This stays within the existing project-hook framework.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Bound prompt-hook payload before exporting to env

run_user_prompt_submit_hooks serializes the full items array into CODE_HOOK_PAYLOAD, and run_hook_command forwards that JSON verbatim as an environment variable; for multimodal prompts, InputItem::Image carries a base64 data URI that can be very large. On typical OS env-size limits, this can make hook process launch fail (E2BIG), so user.prompt_submit hooks silently stop working precisely when users attach screenshots/images. This is a regression in hook reliability for common image workflows and should be fixed by truncating/omitting large fields before env export.

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