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>

plugin/PACKAGING.md

@@ -14,7 +14,7 @@ another one means adding files, not moving or rewriting any.
 | Piece | Where | What it is |
 |---|---|---|
 | **The skills** | `plugin/skills/*/SKILL.md` | Agent Skills (the open SKILL.md format). They state every Scribe reflex in full and name no client. `tests/test_guidance_ownership.py` fails if a skill names a particular client, or references anything outside its own folder. Every client package ships this folder verbatim. |
-| **The MCP server** | `<base URL>/mcp` | HTTP, `Authorization: Bearer <fmcp_ key>`. Its `_INSTRUCTIONS` is a client-neutral index (≤2,000 chars); each tool's description carries its contract; in-band responses (`placement`, `report_back`, `systems_hint`, the duplicate gate, the guessed-id refusal) fire in every client. |
+| **The MCP server** | `<base URL>/mcp` | HTTP, `Authorization: Bearer <fmcp_ key>`. Its `_INSTRUCTIONS` is a client-neutral orientation — the workflow across tools (≤1,600 chars, #4389); each tool's description carries its contract; in-band responses (`placement`, `report_back`, `systems_hint`, the duplicate gate, the guessed-id refusal) fire in every client. |
 | **The adapter API** | `<base URL>/api/plugin/*` | Plain `GET` endpoints any client's hooks can call with the same key (read scope is enough): `context` (live session state), `retrieve` (rules, preferences and notes for a message), `prior-art` (records and shape-ledger hints for code being written), `tool-rules` (rules for a command about to run), `report-check` (records a completion-report check and returns the reason for a block), `processes` (stored Processes to expose as skills). |
 | **The API key** | Scribe → Settings → API Keys | One `fmcp_` key per install. Read scope for hooks; write scope for the MCP tools. |