fix(agents): pass multi-root sources to Chat, Loop, and Graph

[im-ian]

Summary

Ensure Claude Code and Codex receive every source root in a multi-root project across terminal and native agent sessions.

1. Shared root resolution — reuse one sibling-root resolver for PTY and native execution paths.
2. Native agent coverage — pass additional roots through Chat, Loop, and Graph adapters as attached --add-dir arguments.
3. Provider scoping — keep Antigravity, Grok, Ollama, LLM, and custom provider arguments unchanged.
4. Regression coverage — verify spaces and colons in direct CLI path arguments plus primary/source root symmetry.

Expected impact

Flow	Before	After
Terminal Claude/Codex	All project roots visible	All project roots visible
Chat / Loop Claude/Codex	Starting root only	All project roots visible
Graph Claude/Codex	Starting root only	All project roots visible
Other providers	Starting root only	Unchanged

Design notes

• Native CLI invocations receive typed path arguments directly instead of reusing the colon-delimited PTY environment representation.
• Codex source flags stay before the exec subcommand, matching the existing wrapper contract.
• Graph retries and human-input continuations clone the same additional-root set as the original executor.

Test plan

• cargo test --manifest-path src-tauri/Cargo.toml commands::tests::chat_cli_invocation -- --test-threads=1 — 9 passed
• cargo test --manifest-path src-tauri/Cargo.toml commands::tests::source_dirs_env_lists_only_the_session_root_siblings -- --test-threads=1 — 1 passed
• cargo test --workspace --manifest-path src-tauri/Cargo.toml -- --test-threads=1 with Acorn control-session environment overrides removed — passed
• git diff --check — passed

[im-ian]

Merge readiness note: this PR is technically mergeable now.

• GitHub reports MERGEABLE / CLEAN.
• CI is green: Frontend, E2E (shard 1/2), and E2E (shard 2/2) passed.
• Self-review is complete with no blocking findings.
• fix label is applied.

Proceeding to merge because the user explicitly requested the reviewed release fixes to be merged.