feat(shell): add startup_command run in each new pane

[roramirez]

Summary

Adds a shell.startup_command config option. When set, the command is written to every newly spawned interactive pane (new tab or split) right after the shell starts — useful for e.g. ssh myserver. The command is intentionally not re-run for panes reconstructed from a saved session on restore.

Changes

• src/config/mod.rs: add startup_command: Option<String> to ShellConfig (#[serde(default)], defaults to None).
• src/pane_ops.rs: spawn_pane_into gains a run_startup: bool parameter. Interactive callers (spawn_pane, new_tab) pass true; restore callers in src/restore.rs pass false. When true and the config value is Some(non-empty), the command + \n is written to the new pane's PTY.
• src/config/tui_config.rs: add F_STARTUP_COMMAND field (Startup Command) to the in-app config editor, with empty→None mapping in build_config.
• assets/config.toml, README.md, doc/SPEC.md: document the option (commented example + spec table row and prose).
• CHANGELOG.md: add entry under ## [Unreleased] → ### Added.
• Tests: config round-trip (config_test.rs), empty→None / non-empty→Some decision through the config editor (tui_config_test.rs), and updated field-count / index-contiguity / navigation-clamp asserts.

Note on coverage: the empty→None/Some decision and config serialization round-trip are unit-tested. The end-to-end PTY-write path in spawn_pane_into requires a fully constructed App (window + renderer) and is not unit-tested; the write itself uses the existing, already-tested PtySession::write_input.

How to test

1. Set in ~/.config/mmterm/config.toml:

   [shell]
   startup_command = "echo hello-from-startup"

2. Launch mmterm — the initial pane runs the command. Open a new tab (or split) and confirm each new pane also runs it.
3. Enable restore_session, save a session with Ctrl+Q, relaunch — restored panes do not re-run the command.
4. cargo test — config round-trip and decision tests pass; cargo clippy --locked -- -D warnings is clean.