Add a run-level warnings/caveats file for deferred or failed steps

[jdevera]

Found during the fresh-macOS install documented in #20.

Motivation

The fresh-install run produced several situations where a step couldn't fully complete but the right behaviour is to keep going and tell the user to do something later — not to abort. Today those notices either kill the run (dot::step::fatal) or scroll past and get lost.

Proposal

Introduce a run-level warnings/caveats mechanism (conceptually like Homebrew's "Caveats"): a file that any chezmoi script can append to during a run, surfaced as a summary at the very end of chezmoi apply / init.

Use cases that motivated this:

• "Could not reach the hishtory remote — install completed in offline mode; run … later to register." (see hishtory: pre-flight remote reachability check with offline fallback (don't abort the run) #28)
• hishtory history import couldn't complete; retry later. (see hishtory: shell history import never runs / gives no feedback #29)
• Homebrew needed sudo that wasn't primed. (see macOS bootstrap: Homebrew install aborts when sudo isn't pre-authorized #23)
• Any tool that needs a shell reload before it's usable.

Sketch of behaviour

• A helper in home/.chezmoitemplates/dotfiles_bashlib.sh, e.g. dot::warn_later "message", appends to a known file (e.g. ${XDG_STATE_HOME}/dotfiles/run-warnings).
• The file is reset at the start of a run.
• At the end of the run, if non-empty, print a clearly delimited "Caveats / things to finish" block.

This pairs with the hishtory robustness work and the Homebrew-sudo issue, which should write to it instead of failing hard.