refactor(mcp): _INSTRUCTIONS orients the workflow, not a rulebook (#4389)

Spike #4389 read the spec, Claude's docs and a dozen servers: the field is
for how the tools fit together, and the field runs ~600-1,600 characters.
Ours sat at the 2,048 cap as a keyword index that also carried stance.

- JUDGE, REPORT and MISSED leave the index. They fire mid-work, not at
  session start; using-scribe and reporting-back state them in full, and
  `placement`/`report_back` cue reporting in-band. No skill text changes.
- The rest is rewritten as plain practices (1,503 chars) and keeps every
  session-start marker the ownership registry pins.
- INSTRUCTIONS_BUDGET 2000 -> 1600; the three index markers are dropped
  from the registry; the miss-route index test now checks that the index
  keeps what_might_apply and stays off the route.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_instruction_surfaces_agree.py

@@ -69,8 +69,11 @@ def _instructions_text() -> str:
 # Claude Code injects only the first ~2,048 characters of an MCP server's
 # instructions and silently cuts the rest mid-word (#2562: observed live —
 # the previous 20k-char version delivered ~10% of itself, and none of the
-# Systems tagging guidance ever reached a session). 2,000 leaves margin.
-INSTRUCTIONS_BUDGET = 2000
+# Systems tagging guidance ever reached a session). The budget is set below
+# the cap, not at it: the field runs ~600–1,600 characters (spike #4389), and
+# a block pressed against 2,048 is being used for depth that belongs in a
+# skill.
+INSTRUCTIONS_BUDGET = 1600
 
 
 def test_instructions_fit_the_fold():
@@ -181,9 +184,10 @@ def test_the_strength_guard_can_fail():
 # ── Reporting back (milestone 409 step 3) ──────────────────────────────
 #
 # The reporting-back skill carries the shapes, but a skill only helps if it
-# fires. The reflex that sends a session to it is stated in using-scribe; the
-# server's index carries a REPORT line pointing at `placement`, and the in-band
-# cue on update_task is the half that fires on its own in every client. The
+# fires. The reflex that sends a session to it is stated in using-scribe, and
+# the in-band cue on update_task (`placement`, `report_back`) is the half that
+# fires on its own in every client. The server's index carried a REPORT line
+# until #4389: reporting is a mid-work stance, not a session-start practice. The
 # static context was a second plugin-side copy until milestone 410.
 
 REPORT_REFLEX = "report back in a shape the operator can read"