feat(6854): the server says which project a pending act would change, and whether this session may do it

GET /api/plugin/write-scope answers the plugin's fence. The session's project
comes from repo / project_id, as every plugin endpoint reads it. The act is
either:
- a directory (target_repo / target_project_id), for a file edit or a git
  command; or
- an MCP call (tool, plus input: the call's short arguments).

It returns allowed, the other projects the act would change, and the reason
the operator is asked. It is a GET so a read-scoped key still fences file
edits and git.

services/write_scope.WRITE_TARGETS lists every write tool with the arguments
that locate its write: a project id, or a task, note, milestone, System, rule,
relation, trash batch, repo or family decision, each traced to its project. A
write that lands in no project (install settings, user-level records, the
judgments a session is asked for anywhere) has an empty entry. The one
cross-project write allowed is create_task(kind="issue") as a new, unplanned,
open issue. A project the caller cannot read is never named.

tests/test_write_scope.py:
- requires an entry for every write tool;
- requires every declared argument to exist on the tool;
- refuses a locating argument (project_id, task_id, milestone_id and the
  rest) that a write tool takes without declaring it.

The integration test traces each locator from real rows.

Milestone 583, step 2.

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

plugin/PACKAGING.md

@@ -15,7 +15,7 @@ another one means adding files, not moving or rewriting any.
 |---|---|---|
 | **The skills** | `plugin/skills/*/SKILL.md` | Agent Skills (the open SKILL.md format). They state every Grimoire 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 orientation — the workflow across tools (≤1,600 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 adapter API** | `<base URL>/api/plugin/*` | Endpoints any client's hooks can call with the same key. `GET`, so 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), `moment-tools` (which tools reach a mounted moment), `shape-check` (definitions written this turn that nobody has judged), `claim-session` / `release-session` (task claims), `processes` (stored Processes to expose as skills). `POST`, so write scope: `moment` (rules mounted on the moments a tool call reaches), `rule-opened` (a rule was read), `reply-rules` (the finished reply, held for an unopened rule or — when the turn closed a task — for missing completion sections). |
+| **The adapter API** | `<base URL>/api/plugin/*` | Endpoints any client's hooks can call with the same key. `GET`, so 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), `moment-tools` (which tools reach a mounted moment), `shape-check` (definitions written this turn that nobody has judged), `claim-session` / `release-session` (task claims), `write-scope` (which project a pending write lands in, and whether this session may make it), `processes` (stored Processes to expose as skills). `POST`, so write scope: `moment` (rules mounted on the moments a tool call reaches), `rule-opened` (a rule was read), `reply-rules` (the finished reply, held for an unopened rule or — when the turn closed a task — for missing completion sections). |
 | **The API key** | Grimoire → Settings → MCP Access | One `fmcp_` key per install. Write scope covers the MCP tools and every hook; a read key runs only the `GET` hooks. |
 
 ## Added by each client