synapse: a reaction continues its conversation, and the request becomes a record ...

Every Synapse event used to open its own service session, so two messages of
one Telegram chat landed in two unrelated chats. The dispatcher now returns a
`session_key` — derived from the payload according to the user's routing
document — and a later event carrying the same key joins the session that
opened it, within `reaction_session_ttl_minutes` of idle time (0 restores the
old always-fresh behaviour). A session that is still running is never joined:
`create_run` overwrites `state.run`, so a second run would orphan the first
one's subscribers — a busy thread gets a fresh session instead. When the
dispatcher names a different profile for the event, the session's profile is
switched through `set_profile`, never through `save()`, which does not rewrite
`name` or `profile_id`: the thread keeps its name and its history.

What the user reads in the transcript is now only the request. The event
record is a system message (`metadata.source == "synapse_event"`) drawn by a
new SynapseEventNotice.vue in the visual language of the recall badge, and the
framing plus the JSON envelope become a hidden user turn written once, on the
first event of a thread — not on every one. The instructions from Settings
reach the model through a new `current_reaction_instructions` ContextVar and
the `[Reaction session]` block of the system prompt, so they appear in no
transcript at all. The stored system record still has to reach the model, so
ContextBuilder's allowlist becomes `_LLM_SYSTEM_SOURCES = {task_note,
synapse_event}`.

The dispatcher gets a document of its own — how to route, rather than what to
do — stored beside the reaction instructions, versioned separately, and edited
in its own field in the settings panel. `synapse_instruction_versions` gains a
`doc` discriminator, so existing rows keep `'reaction'` and their history
verbatim. The new columns are added by `_MIGRATE`, and the index over `doc`
lives there too: `_DDL` runs first, so an index over a column the migration has
not added yet kills the whole batch on an existing database — which is what
the prod database is. A guard test keeps it that way.

Tests: 1776 passed, 1 skipped (backend), 242 passed (webclient, 28 files).

profiles: restricted profiles for ordinary users, and a role gate that holds ...

`is_admin_only` was checked in one place out of nine and was not read from
config.json at all, so all seven profiles were reachable by every account with
role `user`. The flag now lives in config.json — the file is the baseline, a
`profile_overrides` row still wins on top of it — and one predicate,
`admin_only_blocked`, is the single place the rule is expressed. The nine
surfaces that list, switch to, spawn or resolve a profile all consult it:
`POST /sessions`, the WebSocket, switch_profile, list_profiles, the system
prompt's "Available profiles" block, spawn_agent and the Synapse reaction
runner. The prompt cache is now keyed by (profile, role), so a user's prompt
can never be served an admin's profile list.

The seven existing profiles (developer, discuss, dispatcher, modeler_3d,
navi_code, secretary, server_admin) are marked admin-only. Three new ones take
their place for ordinary users: assistant, designer_3d and coder. They share one
native tool set — ssh_exec, peer, reload_tools, create_mcp_server, test_mcp_tool,
image_view and gmail are withheld — and differ only in system prompt, model and
MCP groups. navi-web's raw `request` group, and the whole of gnexus-creds and
tgclient, are withheld too.

MCP per-user keys gain the missing half of the rule: a server that declares a
`user_key` slot is refused to anyone but an admin who has no personal key, and
is left out of their tool list entirely, instead of quietly falling back to the
owner's credential and appearing as a tool that cannot work. The refusal names
the server and points at Settings.

Also closes `GET /agents/prompts`, which served every profile's system prompt to
anyone, with no user dependency at all.

The accepted residual risk is written down in docs/profiles.md: the working
directory is a convention, not a sandbox.

docs: synapse integration guide, special sessions filter, new endpoints ...

- docs/synapse.md: full guide — gateway, reaction runner pipeline,
  notify tool, per-user settings, service sessions in the UI
- index/tools/config/api: entry point + built-in tools + env vars +
  GET /sessions special param + synapse REST/webhook endpoints