feat!: require session names and rework wait, send, and kill flags

[kylecarbs]

Summary

Breaking CLI refinements based on usage feedback:

• wait flags reworked: --idle is now a plain flag that fires once output has settled for 2 seconds. --text <t> replaces --for <t>. --timeout <dur> is the only flag that takes a duration.
• send syntax reworked: the session name is now positional (boo send <name> --text <t>), text moved behind --text, and -s is removed. --key, --enter, and --stdin are unchanged; stdin remains the default when neither --text nor --key is given.
• Session names are required everywhere: attach, send, peek, wait, and kill now exit 2 with usage when no name is given. Most-recent-session fallback is removed; unambiguous prefix matching is kept.
• exorcise removed: boo kill --all is the only end-everything path and prints the names it kills.
• Duration parsing: accepts ms, s, m, h/hr, and d units (e.g. --timeout=4h, --timeout=10d), with overflow rejected.

Both --flag value and --flag=value spellings are supported for all value-taking flags.

Testing

• zig build test-all --summary all: 67/67 pass (Debug and ReleaseSafe)
• zig fmt --check build.zig build.zig.zon src test: clean
• Manual smoke: new send/wait syntax, --text=/--timeout= forms, bare attach/kill/peek/wait exit 2, removed exorcise exits 2, kill --all, 10d/4hr durations parse, 9x rejected.

Decision log

• Idle threshold fixed at 2s (idle_settle_ms const in main.zig): a flag-with-value would reintroduce the duration ambiguity the rework removes; 2s matches the agent polling loop in the docs.
• flagValue helper: one parser supports both --flag value and --flag=value so all commands behave identically.
• Prefix matching kept: requiring names everywhere makes typing full names common, and unambiguous prefixes (exit 3 on ambiguity) keep that cheap without the magic of most-recent fallback.
• pickMostRecent and the Pick enum deleted rather than parked: no remaining callers, and kill --all covers the bulk-cleanup use case exorcise served.

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