[bnivanov-true]

Summary

Adds an optional due day to a task, as proposed in #270. Calendar dates only (YYYY-MM-DD, no time of day).

• tsk add --due, tsk edit --due / --clear-due, and due on a plan item. A bad day is refused with invalid-due (exit 1, store untouched).
• tsk list --sort due: earliest first, undated last. SortBy::Due is shared, so the board's sort picker and config.toml sort = "due" accept it too.
• The task page and text rows show the day; --json carries due only when set. A due edit appears in the paper trail as "due edited".
• No new statuses; due does not affect after or dispatch.
• Docs, CHANGELOG, tsk-cli and tsk-coordinator skills (1.7.0, hash pins refreshed).

Decision for the maintainer: store format 16

This bumps the store format to 16: MIGRATIONS gains v6 to v7 plus identity steps for the Dispatch dev formats v7 to v15, and DEV_FORMAT_VERSIONS is deleted. I first tried staying on v6 (the optional-field precedent in 50280d4), but that commit landed before v0.13.0 shipped. After release, 0.13.0 can silently drop a trashed dated task it cannot parse, and no tsk.json.v6 copy exists to roll back to. With the bump, 0.13.0 refuses a format 16 store (as it does v5 stores today) and the rollback is the tsk.json.v6 copy.

Questions:

• Is format 16 the number you want, or would you rather renumber or keep the dev range?
• If you prefer no bump, the diff shrinks by the store.rs and store_persist.rs changes; say so and I'll rework it.

Not in this PR

A due editor on the task page, a quick-add token, due on board rows, and priority, importance or urgency. Say if you want any of them as follow-ups.

Test plan

• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo test
• cargo build --release
• cd site && npm ci && npm test && npm run build
• tests/cli_due.rs: all 14 tests fail on main (8d714a2) and pass on this branch
• Smoke: migrated a copy of a real v6 board, set, sorted, cleared and refused dates with the release binary; read the task page in a standalone board pane
• Manual herdr open-board smoke: not run (I don't use herdr)
• npm run test:parity (Playwright): not run
• Mutation testing after the format bump: not rerun (the pre-bump diff had 2 misses and 7 timeouts)

[vercel]

@bnivanov-true is attempting to deploy a commit to the smarzban Team on Vercel.

A member of the Team first needs to authorize it.