feat(session): support --agent-cmd flag on wt session add

[ukanga]

Closes #11

Summary

• Adds --agent-cmd <CMD> to wt session add for a one-off override of session.agent_cmd. Applies to both panes mode and windows mode when creating a new agent pane/window.
• Override does not mutate persisted config and is not retroactively applied to an existing tmux window/session — an explanatory note is printed in that case.
• Precedence (highest wins): --agent-cmd > .wt.toml > ~/.wt/config.toml > default.

Changes

• src/config.rs: SessionConfig::with_agent_cmd_override returns a clone with agent_cmd swapped only when an override is provided.
• src/session_cmd.rs: clap agent_cmd: Option<String> on SessionAction::Add, threaded through run_session into both cmd_session_add_panes and cmd_session_add_windows; setup uses the overridden SessionConfig.
• src/main.rs: CLI parsing tests for both with/without --agent-cmd.
• tests/tmux_test.rs: configure git identity in setup_test_repo so the initial empty commit succeeds in CI without a global git identity.
• README.md: CLI reference, example, dedicated paragraph on one-off semantics, and updated precedence rule.

Test plan

• cargo build clean
• cargo test — 74 pass (51 lib + 11 bin + 3 integration + 9 which); 4 tmux-dependent tests remain #[ignore] as before; 0 failures
• CLI parsing covered with and without the flag
• Override application covered for both panes and windows mode (via effective_session_config)
• No-mutation guarantee covered (test_effective_session_config_override_is_one_off)
• Full precedence chain covered (test_full_precedence_chain_cli_beats_local_beats_global_beats_default)
• Manual smoke: wt session add demo --agent-cmd "opencode --continue" in panes mode and wt session --mode windows add demo --agent-cmd "opencode --continue" in windows mode

[pld]

looks good! seems reasonable that --agent-cmd works when not in a pane or window as well, maybe for some reason you want to run it in a new term emu not in the current tmux work flow?

[pld]

this duplicates 293-297

[ukanga]

looks good! seems reasonable that --agent-cmd works when not in a pane or window as well, maybe for some reason you want to run it in a new term emu not in the current tmux work flow?

Is this the mode that creates the worktree and drops you into a new term emu in that worktree? Could the same apply where you may not need the term, but need the agent at that time? The use cases where this may have been relevant have been:

• I need to investigate an issue and will likely be using an agent to do the investigation
• I want to scope an issue, flesh it out, then create the relevant issue on GitHub, for example, that would be picked up later. Using an agent here is most likely

In most scenarios, I have ended up firing an agent for one reason or another, since the task at hand required it. In most cases, the task was investigative or scoping, not implementation. But it could be if the task is quick and small.

[pld]

looks good! seems reasonable that --agent-cmd works when not in a pane or window as well, maybe for some reason you want to run it in a new term emu not in the current tmux work flow?

Is this the mode that creates the worktree and drops you into a new term emu in that worktree? Could the same apply where you may not need the term, but need the agent at that time? The use cases where this may have been relevant have been:

* I  need to investigate an issue and will likely be using an agent to do the investigation

* I want to scope an issue, flesh it out, then create the relevant issue on GitHub, for example, that would be picked up later. Using an agent here is most likely

In most scenarios, I have ended up firing an agent for one reason or another, since the task at hand required it. In most cases, the task was investigative or scoping, not implementation. But it could be if the task is quick and small.

Yep both of those scenarios, we're seeing this on our team, in work with one agent, you encounter an out of scope issue that you'd want to spawn another agent to fix in an interactive session, that last bit being the key. Going to merge this now since this is out of scope, but if could make a nice follow-up if we keep encounter that situation