hooks: ship three commented examples, install them, document the contract

Add hooks/on_start, hooks/on_done and hooks/on_quit. They are named
after the hooks themselves so copying one into ~/.config/ttym/hooks/ is
the entire installation step, and each carries its argument contract in
a header comment.

  on_start  sets the terminal window title to what is running
  on_done   desktop notification plus a sound, both backgrounded, with
            the first available of paplay/aplay/mpv/ffplay
  on_quit   records abandoned sessions to a TSV under XDG_STATE_HOME and
            restores the terminal title

make install places them in PREFIX/share/doc/ttym/hooks; make uninstall
removes them and both directories; make dist ships hooks/ with the
executable bits intact.

The man page's HOOKS section now documents what the code actually does
rather than what could be assumed from the call sites:

  - $2 differs per hook: requested duration on on_start, full duration
    on on_done, and time actually elapsed -- excluding time spent paused
    -- on on_quit. An abandoned 25m countdown reports what really ran.
  - $3 is always passed, so it is never unset.
  - hooks block: timer forks and waitpid()s, so on_done delays the
    completion bell and the alert loop. Background anything slow.
  - stdin, stdout and stderr all go to /dev/null, which is why printing
    is pointless and cannot corrupt the redrawn progress line.
  - execvp(3) means a script needs a #! line.

README gains a Hooks section. All three examples pass sh -n and
shellcheck, and were tested end to end through the documented flow:
make install to a staging prefix, copy into a config dir, run.

Claude-Session: https://claude.ai/code/session_01APLBs8RB1FcUaC4viVkzbP

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