feat: add justification arg to prefix_rule() in *.rules

[bolinfest]

Adds an optional justification parameter to the prefix_rule() execpolicy DSL so policy authors can attach human-readable rationale to a rule. That justification is propagated through parsing/matching and can be surfaced to the model (or approval UI) when a command is blocked or requires approval.

When a command is rejected (or gated behind approval) due to policy, a generic message makes it hard for the model/user to understand what went wrong and what to do instead. Allowing policy authors to supply a short justification improves debuggability and helps guide the model toward compliant alternatives.

Example:

prefix_rule(
    pattern = ["git", "push"],
    decision = "forbidden",
    justification = "pushing is blocked in this repo",
)

If Codex tried to run git push origin main, now the failure would include:

`git push origin main` rejected: pushing is blocked in this repo

whereas previously, all it was told was:

execpolicy forbids this command

[joshka-oai]

LGTM

I note that this doesn't have a UI snapshot test change.
This would make it clear how these messages end up being rendered by the TUI.
I'm assuming that we probably don't have that plumbed in yet, but likely should.
(not a blocker for this PR - but possibly worth a quick follow up)

[joshka-oai]

Add a note about how this can be used to present alternatives here (like mentioned in the doc comments).