feat: add chat context pinning storage and push trigger (#26385)

Foundation for the Workspace Context Sources RFC (phase 3). The agent
push (#25983) and coderd snapshot storage (#26145) already persist
per-agent context snapshots; this PR lands the **chat-side storage**
plus the **`agentapi` push trigger** that a follow-up will use to read
them. It does **not** touch `chatd` and changes no behavior — nothing
wires an implementation yet.

## What changed

- Adds four nullable columns to `chats` — `context_aggregate_hash`,
`context_dirty_since`, `context_dirty_resources`, and `context_error` —
and rebuilds the `chats_expanded` view.
- Adds three queries — `SetChatContextSnapshot`,
`HydrateAgentChatsContext`, `MarkChatsContextDirtyByAgent` — with
`dbauthz` wrappers and `audit` entries. They are store-interface methods
covered by a Postgres test (`TestChatContextHydration`).
- Adds the `agentapi.ContextDirtyMarker` interface and invokes it inside
the `PushContextState` transaction, publishing collected events only
after commit.

## Intentionally inert

There are **no production callers** of the three queries and **no
implementation** wired for `ContextDirtyMarker`, so the push trigger is
dormant. This is deliberate: the PR is the durable storage/query
foundation only.

The actual integration — the `chatd` implementation that
hydrates/dirties chats and backs a refresh endpoint, consuming the
pinned context in prompt building, the rich SDK types + UI, and retiring
the live per-turn pull — lands as a single follow-up PR. Splitting this
way keeps the schema/query layer reviewable on its own and keeps the
integration whole in one place.

Refs #25983, #26145.

<details>
<summary>Decision log</summary>

- **Columns over a side table.** The four `chats` columns are the
durable model (accepting the one-time `chats_expanded` view/CTE churn).
`last_injected_context` is deliberately left untouched — it is
load-bearing for the live per-turn context pull.
- **Keep `agentapi`, drop `chatd`.** The earlier revision wired the
hydrate/dirty implementation through `chatd` and added a `PUT
/chats/{chat}/context` refresh endpoint. Those were removed so this PR
is pure foundation; `agentapi` defines the trigger + interface (it does
not import `chatd`), and the `chatd` implementation arrives with the
full integration.
- **No new experiment flag.** The columns are dark and unread by prompt
building.
- **Authz.** The new query wrappers authorize chat updates under the
chat RBAC object / `ResourceChat`, consistent with the existing system
chat mutators.

</details>

---

🤖 Generated by Coder Agents on behalf of @kylecarbs.

coderd/database/models.go

@@ -4797,6 +4797,10 @@ type Chat struct {
 	GroupACL                 ChatACL               `db:"group_acl" json:"group_acl"`
 	OwnerUsername            string                `db:"owner_username" json:"owner_username"`
 	OwnerName                string                `db:"owner_name" json:"owner_name"`
+	ContextAggregateHash     []byte                `db:"context_aggregate_hash" json:"context_aggregate_hash"`
+	ContextDirtySince        sql.NullTime          `db:"context_dirty_since" json:"context_dirty_since"`
+	ContextDirtyResources    pqtype.NullRawMessage `db:"context_dirty_resources" json:"context_dirty_resources"`
+	ContextError             string                `db:"context_error" json:"context_error"`
 }
 
 type ChatDebugRun struct {
@@ -4983,6 +4987,14 @@ type ChatTable struct {
 	RetryStateVersion        int64                 `db:"retry_state_version" json:"retry_state_version"`
 	RunnerID                 uuid.NullUUID         `db:"runner_id" json:"runner_id"`
 	RequiresActionDeadlineAt sql.NullTime          `db:"requires_action_deadline_at" json:"requires_action_deadline_at"`
+	// Aggregate hash of the agent context snapshot this chat is pinned to. NULL until first hydrated; compared against the agent's latest snapshot hash to detect drift.
+	ContextAggregateHash []byte `db:"context_aggregate_hash" json:"context_aggregate_hash"`
+	// Set when an agent push changes the pinned hash; cleared on refresh. NULL means clean.
+	ContextDirtySince sql.NullTime `db:"context_dirty_since" json:"context_dirty_since"`
+	// Deterministic prefix of resources that changed since the pinned hash. Reserved for the dirty diff; left NULL until the UI phase populates it.
+	ContextDirtyResources pqtype.NullRawMessage `db:"context_dirty_resources" json:"context_dirty_resources"`
+	// Snapshot-level error copied from the pinned snapshot (count cap exceeded, watcher degraded, etc.). Empty when healthy.
+	ContextError string `db:"context_error" json:"context_error"`
 }
 
 type ChatUsageLimitConfig struct {