fix(when): report a --when the read-back cannot place as not confirmed

[ryanlewis]

The read-back's whenCheck.holds treats a value placeWhen cannot model as held, so add, project add and edit reported "confirmed" for a weekday name, a weekday or evening with a time, or a phrase Things understood, after checking only that Things applied the write.

Cut from origin/main; it does not depend on #449.

Changes

• whenPlaced (when.go) says whether a value, as sent, is one placeWhen models: anytime, someday, today, evening, tomorrow, empty, HH:MM, a date, or a date and time.
• add / project add: when the item is found and the --when is not one of those, warn on stderr with where the item is and report it not confirmed, exit 0, with the item's uuid.
• edit / project edit: once the read-back sees the edit applied, the same. The existing phrase-only edit path (reason: when) is unchanged, and the new case reuses its output shape (printUnconfirmedEdit) and reason.
• Values the read-back does place are judged as before. A phrase Things ignored (no start date) still warns and prints the item, as before.
• Docs: agents.md, commands.md, SKILL.md.

Before / after

things add "Buy oat milk" --when friday, which Things files on Friday:

• before: the item printed as things show would (confirmed).
• after: stderr warning: the CLI cannot check where --when "friday" files an item, so the to-do is not confirmed; it was created scheduled for 2026-10-16. Run things show to check. stdout Created, not confirmed (--when not checked): "Buy oat milk" (<uuid>). Under --json: {"uuid": "<uuid>", "title": "Buy oat milk", "confirmed": false, "reason": "when"}.

things edit <uuid> --when evening@6pm --notes x, once applied: Applied, not confirmed (--when not checked): "…" (<uuid>), with the same JSON shape.

Tests

TestAddWhenUnplaced (friday, friday@18:00, evening@6pm, for to-dos and projects in both output modes, plus a date@time that is still confirmed), TestEditWhenUnplaced, and TestAddWhenPhraseIgnored updated: a phrase Things understood is now unconfirmed. make test and make lint pass.