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

plugin/skills/using-scribe/SKILL.md

@@ -64,11 +64,13 @@ Two constraints on *how* that's achieved:
 3. **Update over duplicate.** When recording, prefer updating an existing
    note/rule/task over creating a new one. Search first; revise what's there.
 
-4. **Plans live in Scribe.** For non-trivial work call `start_planning(project_id,
-   title)` FIRST — it creates a milestone whose `body` holds the design; each
-   step is its own task under that milestone (`create_task(milestone_id=...)`),
-   progress goes in work-logs (`add_task_log`). Read it back with `get_milestone`.
-   Do not write plans/specs to local `.md` files.
+4. **When you plan, plan in Scribe.** Work with an *arc* — several steps toward
+   one goal — gets a plan, and a plan is a milestone: `start_planning(project_id,
+   title)` creates one whose `body` holds the design, each step is its own task
+   under it (`create_task(milestone_id=...)`), progress goes in work-logs
+   (`add_task_log`). Work without an arc (a fix, a one-file change, a question)
+   is just a task — don't wrap it in a milestone. Either way, do not write
+   plans/specs to local `.md` files. See the **writing-plans** skill.
 
 5. **Keep state honest.** Set a task `in_progress` when you start it, `done` the
    moment it's complete; log progress as you go.