Decouple planning→execution flip across task lifecycle commands

[bborbe]

What

Makes the task lifecycle four deliberate, non-overlapping steps: work-on → plan-task → execute-task → complete-task.

Why

Previously two commands flipped the same planning → execution transition: plan-task flipped it itself, and work-on* auto-chained execute-task. In the auto-chain plan-task always won the flip, so execute-task ran as a silent no-op and never acted as the real gate.

Changes

• plan-task — validates and hands off; never flips phase. Ends at planning, signals /execute-task.
• execute-task — the sole command that flips planning → execution (unchanged gate logic).
• work-on / work-on-task / work-on-goal — stop auto-chaining plan-task/execute-task; each prints an explicit plan → execute → complete next-step signal.
• Updated docs/task-writing.md phase table + README.md.

work-on-task/work-on-goal kept in parallel with work-on (canonical) per current direction.

Verify

make precommit green (tests, lint, version alignment, CHANGELOG structure).

[ben-s-pull-request-reviewer]

{
  "verdict": "approve",
  "summary": "Pure slash-command documentation refactor — all 7 Plan concerns verified correct in diff. plan-task no longer flips phase (now hands off to execute-task), execute-task is the sole planning→execution gate, work-on-task/work-on-goal/work-on all print next-step signals instead of auto-chaining, and task-writing.md phase table reflects the new single-responsibility model accurately. No Go code changed; no test coverage gaps introduced.",
  "comments": [],
  "concerns_addressed": [
    "correctness: plan-task no longer flips phase — Step 7 changed to 'hand off to execute-task (no phase flip)', removed `vault-cli task set phase execution`",
    "correctness: execute-task is now the sole gate for planning→execution flip — lifecycle section updated to 'the sole gate'",
    "correctness: work-on-task Phase 5 changed from auto-sharpen+auto-gate to next-step signal — prints plan/execute/complete commands without invoking them",
    "correctness: work-on-goal has new Phase 3 'Signal next steps' — mirrors work-on-task Phase 5 correctly, no auto-chain",
    "correctness: work-on.md Phase 3 updated to 'Next-step signal' — both task and goal routes print signal without auto-invoke",
    "correctness: docs/task-writing.md phase table updated — plan-task 'hands off to /execute-task — never flips phase', execute-task is 'the sole command that flips planning→execution'",
    "tests: no test files changed — correctly reflects that existing tests cover unchanged behavior (this is a docs-only change to slash command definitions)"
  ]
}