fix(instructions): stop mandating a milestone for every non-trivial task

Four product surfaces told the agent to call start_planning FIRST for any
"non-trivial" work, while a fifth — the milestone bullet four lines up in the
same file — had the criterion right: use one when the work has an arc. The
loudest surface won, so sessions wrapped bug fixes and one-file changes in
milestones that never meant anything.

Mandating one project shape is what rule #115 forbids: some projects are
milestone-shaped, others are a flat task list and always will be.

Now the arc test is stated ONCE in full, in writing-plans, along with what to
do when there is no arc (a task, driven by status and work-logs). The other
surfaces name it and defer:

- writing-plans/SKILL.md gains a "first decide whether this work wants a plan"
  section; its frontmatter trigger is the arc, not "non-trivial"
- using-scribe reflex #4 points at the skill instead of restating it
- server.py's Plan bullet adopts the milestone bullet's own criterion
- server.py's planning paragraph drops from 11 lines to 6: it keeps the claim
  MCP instructions should make (a plan's HOME is a milestone, not a local .md)
  and drops the how, which the skill carries
- start_planning's docstring gains the when

Also removed "call start_planning FIRST — before any brainstorming, design, or
plan-writing skill runs." That was the server asserting priority over the skill
layer. Tools describe what they do; skills decide when they apply.

The structural point outlasts the wording: a surface that restates a rule is a
surface that will eventually contradict it, and nothing checks prose against
prose.

Closes #2322.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

src/scribe/mcp/tools/tasks.py

@@ -276,6 +276,12 @@ async def add_task_log(task_id: int, content: str) -> dict:
 async def start_planning(project_id: int, title: str) -> dict:
     """Begin a plan in Scribe (the preferred home for plans — not a local .md file).
 
+    Reach for this when the work has an ARC — several steps toward one goal,
+    worth tracking as a unit. Work without one (a fix, a one-file change, a
+    question answered) is a task, not a plan: create_task, drive its status, and
+    record progress with add_task_log. A milestone holding a single step is
+    ceremony, and it leaves the project with a plan that never meant anything.
+
     Creates a MILESTONE that IS the plan: its `body` is seeded with a design
     template (Goal/Approach/Verification) under the given project, and the call
     returns it together with the project's applicable Rulebook rules and brief