refactor(fleet-launcher): introduce CliConvention trait

[NagyVikt]

Summary

• New CliConvention trait, implemented on CliKind (one match arm per variant) to replace the inlined 4-way match in build_command_line()
• build_command_line() shrinks from ~32 lines to a single delegating call
• CliKind::binary() and CliKind::home_env() kept as inherent methods that delegate to the trait, so existing callers (bin/spawn.rs, heartbeat.rs) compile unchanged

Trait

pub trait CliConvention {
    fn binary(&self) -> &'static str;
    fn home_env(&self) -> &'static str;
    fn command_line(&self, prompt: Option<&str>, extra_args: &[String]) -> String;
}

Chose enum-impl over struct-per-CLI: CliKind is Copy, already wired into the public API and heartbeat.rs, and all four CLIs are compile-time known — a Box<dyn CliConvention> registry would add an allocation per command build for zero call-site benefit. The trait still provides a swap-in seam if dynamic registration is needed later.

Behavior

Identical for all 4 CLIs. Verified by the existing tests (build_command_line_per_cli_conventions, cli_home_envs_match_each_cli, cli_parse_accepts_aliases, write_launch_script_emits_exports_cd_and_exec):

• codex: bare positional prompt — codex hello --flag 'with space'
• claude: still emits --dangerously-skip-permissions before the prompt — claude --dangerously-skip-permissions 'review this'
• gemini: still piped via stdin — printf '%s' hi | gemini
• claw: bare positional prompt, no auto-injected bypass flag — claw 'ship it'
• Home env vars unchanged: CODEX_HOME, CLAUDE_CONFIG_DIR, GEMINI_CLI_HOME, CLAW_CONFIG_HOME

Stack

Targets feat/fleet-launcher (#158) which introduces the crate. GitHub will auto-retarget this PR to main once #158 merges.

Test plan

• cargo check -p fleet-launcher — clean
• cargo test -p fleet-launcher — 12/12 pass
• No other workspace crate depends on fleet-launcher (greped — fleet_launcher:: is only referenced inside this crate's own bin/ and heartbeat.rs)