feat(hook): print the snippet a shell evaluates to follow an apply

[torabit]

Implements docs/hook.md, which #79 decides. Stacked on #79 — the base is
torabit/docs/hook, so this diff is the implementation only. Merge #79 first.

Part of #77.

What lands

vanadis hook <zsh|fish|bash> prints a snippet to evaluate. It sources the output of every
target marked shell, and registers a prompt hook that sources it again when that file's mtime
moves. shell is a new [[targets]] key; a value that names none of the three is a config
error rather than a target that silently never sources.

shell	mtime	prompt hook	forks per prompt
zsh	zstat from zsh/stat	add-zsh-hook precmd	none
fish	path mtime	--on-event fish_prompt	none
bash	stat, written in at generation time	PROMPT_COMMAND	one

hook reads config.toml and nothing else: no theme resolved, no state file read, themes/
not scanned. Every failure leaves stdout empty, so a config that will not load cannot leave half
a snippet in the user's shell.

How it is tested

tests/hook.rs starts a real interactive shell, feeds it the snippet, and reads what the shell
prints back. Asserting the snippet as a string would establish that the generator did not change
and nothing about whether zsh can parse the result. Five behaviours per shell: sources at
evaluation, sources again after a cycle, one hook after a second evaluation, the exit status
survives, and an apply --only naming no shell target re-sources nothing.

CI installs zsh and fish. The tests skip a shell that is not installed and refuse to skip when
CI is set, so deleting the install step fails the run instead of quietly passing it.

zsh and bash were also driven by hand on this machine before the tests were written. fish is not
installed here, so the fish snippet is first executed by CI on this PR.

Also in this PR

• docs/examples/config.toml: the zsh palette target, the reference config's one target read
  from the environment, now carries shell = "zsh".
• skills/vanadis/references/reload.md and SKILL.md: the fzf row now points at the hook
  instead of ending at exec zsh, with a section on marking a target and the rc line. Held back
  from docs(hook): decide how a sourced target follows an apply #79 because the skill ships to users and should not name a command a released binary does
  not have.
• docs/hook.md: the fish spelling, vanadis hook fish | source, which is what fish users write
  for every tool in this shape.

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