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/querier.go

@@ -994,6 +994,11 @@ type sqlcQuerier interface {
 	GetWorkspacesByTemplateID(ctx context.Context, templateID uuid.UUID) ([]WorkspaceTable, error)
 	GetWorkspacesEligibleForTransition(ctx context.Context, now time.Time) ([]GetWorkspacesEligibleForTransitionRow, error)
 	GetWorkspacesForWorkspaceMetrics(ctx context.Context) ([]GetWorkspacesForWorkspaceMetricsRow, error)
+	// Stamps the pinned hash and error on every not-yet-hydrated chat for
+	// an agent (context_aggregate_hash IS NULL). Runs as a side effect of
+	// an agent push so chats created before the agent was ready pick up the
+	// snapshot without a dirty event. Does not bump updated_at.
+	HydrateAgentChatsContext(ctx context.Context, arg HydrateAgentChatsContextParams) error
 	// Increments generation_attempt and returns the resulting value.
 	IncrementChatGenerationAttempt(ctx context.Context, id uuid.UUID) (int64, error)
 	InsertAIBridgeInterception(ctx context.Context, arg InsertAIBridgeInterceptionParams) (AIBridgeInterception, error)
@@ -1175,6 +1180,12 @@ type sqlcQuerier interface {
 	// allocate a new snapshot version in one round trip.
 	LockChatAndBumpSnapshotVersion(ctx context.Context, id uuid.UUID) (Chat, error)
 	MarkAllInboxNotificationsAsRead(ctx context.Context, arg MarkAllInboxNotificationsAsReadParams) error
+	// Flips active, already-hydrated chats for an agent to dirty when the
+	// agent's latest snapshot hash differs from the chat's pinned hash. The
+	// pinned hash is intentionally left untouched; the refresh endpoint
+	// re-pins it. Returns the chats that transitioned so the caller can
+	// emit watch events after the transaction commits.
+	MarkChatsContextDirtyByAgent(ctx context.Context, arg MarkChatsContextDirtyByAgentParams) ([]MarkChatsContextDirtyByAgentRow, error)
 	OIDCClaimFieldValues(ctx context.Context, arg OIDCClaimFieldValuesParams) ([]string, error)
 	// OIDCClaimFields returns a list of distinct keys in the the merged_claims fields.
 	// This query is used to generate the list of available sync fields for idp sync settings.
@@ -1219,6 +1230,11 @@ type sqlcQuerier interface {
 	// for the table.
 	// The CTE and the reorder is required because UPDATE doesn't guarantee order.
 	SelectUsageEventsForPublishing(ctx context.Context, now time.Time) ([]UsageEvent, error)
+	// Pins a single chat to the supplied context snapshot hash and error
+	// and clears any dirty marker. Used by chat-create hydration and the
+	// refresh endpoint. Does not bump updated_at: context pinning is
+	// background state and must not reorder chat lists.
+	SetChatContextSnapshot(ctx context.Context, arg SetChatContextSnapshotParams) error
 	SoftDeleteChatMessageByID(ctx context.Context, id int64) error
 	SoftDeleteChatMessagesAfterID(ctx context.Context, arg SoftDeleteChatMessagesAfterIDParams) error
 	SoftDeleteContextFileMessages(ctx context.Context, chatID uuid.UUID) error