docs(hook): decide how a sourced target follows an apply

[torabit]

Closes #77.

docs/hook.md. The document, not the implementation: the issue carries the design label, and
CLAUDE.md resolves a design issue to a document under docs/ before code is written. The
implementation follows in its own PR.

What is decided

vanadis hook <zsh|fish|bash> prints a snippet to eval, the shape direnv, mise, starship
and zoxide already have. hook and not init, because vanadis init is taken by the command
that turns an existing config file into a template.

A per-target shell key, beside reload. A shell name rather than a boolean, so a zsh
target and a fish target sit side by side and each hook sources its own. Not a top-level
table, which would be a second place a target's name is written.

The change signal is the output's own mtime. A missing output sources nothing and is not an
error, because a prompt hook says whatever it says forever. The resolution is one second, and
the gap that leaves is recorded rather than closed.

The paths are baked in. A config parse per prompt is a vanadis process per prompt, which is
what docs/config.md already refused for get. Re-eval is the answer to a stale snippet, and
a baked snippet still works in a shell whose $PATH has lost vanadis.

zsh and fish detect a change with no fork (zstat, path mtime); bash spends one
stat, written into the snippet at generation time from the system vanadis is on, so there is
no runtime branch. The fork-free bash alternative ([[ output -nt stamp ]]) is rejected in the
document: the stamp has to be per shell process, and removing it needs the one EXIT trap the
user also has.

Two invariants for the snippet: a second eval leaves one hook registered, and the hook
returns the exit status it found, so a prompt that shows the last command's status is not lied
to.

Also in this PR

The zsh row of the reload table in docs/config.md now says what does reach it, and the
document is cross-referenced from the header.

Deliberately not in this PR

The same row in skills/vanadis/references/reload.md, and the CI that runs the snippet under
each shell. The skill ships to users; it should not name vanadis hook before a released binary
has it. Both land with the implementation.

🤖 Generated with Claude Code

https://claude.ai/code/session_011wxMB7uC4uxSJ2dfHi8ucf