[alexanderdavidsen]

Closes #58.

What

Adds time-of-day support to due dates, which were previously whole-day deadlines only.

New accepted formats (in addition to YYYY-MM-DD, today, tomorrow):

• ISO date-time — YYYY-MM-DDTHH:MM[:SS] and YYYY-MM-DD HH:MM[:SS]
• Relative offsets — today+HHMM, tomorrow+HHMM (that day at HH:MM), and now+HHMM (HH hours / MM minutes from now). The colon is optional (14:30 == 1430).
• now keyword

Behaviour

• Due dates with a time render as YYYY-MM-DD HH:MM and are classified time-granularly — they flip to Overdue once the instant passes.
• Whole-day deadlines (local midnight) keep date-granular behaviour, so due:today stays due today all day (no regression).

Entry-path nuance

A due: token is whitespace-split, so it can't contain a space — the token form uses the T separator (due:2026-07-01T14:30). The --due flag accepts the quoted space form too (--due @3 "2026-07-01 14:30").

Touched

• taskbook-common/src/due.rs — parsing, formatting, status (core change)
• CLI / TUI / MCP help text + schema descriptions
• CLAUDE.md docs

Test plan

• cargo test --workspace — green (new unit tests cover ISO date-time, space separator, seconds, midnight-renders-as-date, today+/tomorrow+/now+ offsets, invalid offsets, and time-granular vs whole-day status)
• cargo clippy --workspace --all-targets — clean
• cargo fmt --all --check — clean

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