feat(coderd/database): add chat_context_resources table (#26430)

Adds chat_context_resources: a per-chat pinned copy of the agent context
resources a chat is hydrated against. The agent-side table
(workspace_agent_context_resources) is last-writer-wins with no history,
so a chat copies its resources at hydration/refresh to keep a stable view
while the agent drifts.

Schema foundation only (no queries/dbauthz/prepareGeneration/SDK yet).
chat_id FK ON DELETE CASCADE for cleanup parity; no agent FK so the pin
survives agent replacement; PK (chat_id, source); reuses the 000522 enum
types.

coderd/database/migrations/testdata/fixtures/000525_chat_context_resources.up.sql

@@ -0,0 +1,82 @@
+-- Pinned context resources covering each non-reserved body kind plus a
+-- non-OK status. The earlier chat fixtures already insert at least one row
+-- into chats; we attach the resources to the first such chat (ordered
+-- deterministically) so migration tests see a non-empty
+-- chat_context_resources table without hard-coding a specific chat ID.
+INSERT INTO chat_context_resources (
+    chat_id,
+    source,
+    body_kind,
+    body,
+    content_hash,
+    size_bytes,
+    status,
+    error,
+    source_path
+)
+SELECT
+    c.id,
+    v.source,
+    v.body_kind::workspace_agent_context_body_kind,
+    v.body::jsonb,
+    decode(v.content_hash, 'hex'),
+    v.size_bytes,
+    v.status::workspace_agent_context_resource_status,
+    v.error,
+    v.source_path
+FROM (
+    SELECT id FROM chats ORDER BY created_at, id LIMIT 1
+) AS c
+CROSS JOIN (
+    VALUES
+        (
+            '/home/coder/workspace/AGENTS.md',
+            'instruction_file',
+            '{"content":"aGVsbG8="}',
+            '1111111111111111111111111111111111111111111111111111111111111111',
+            5::bigint,
+            'ok',
+            '',
+            ''
+        ),
+        (
+            '/home/coder/workspace/.agents/skills/example/SKILL.md',
+            'skill',
+            '{"meta":"LS0tCm5hbWU6IGV4YW1wbGUKLS0tCmJvZHk=","name":"example","description":"Example skill"}',
+            '2222222222222222222222222222222222222222222222222222222222222222',
+            32::bigint,
+            'ok',
+            '',
+            '/home/coder/workspace'
+        ),
+        (
+            '/home/coder/workspace/.mcp.json',
+            'mcp_config',
+            '{}',
+            '3333333333333333333333333333333333333333333333333333333333333333',
+            128::bigint,
+            'ok',
+            '',
+            ''
+        ),
+        (
+            'mcp:echo',
+            'mcp_server',
+            '{"server_name":"echo","description":"echoes input"}',
+            '4444444444444444444444444444444444444444444444444444444444444444',
+            256::bigint,
+            'ok',
+            '',
+            '/home/coder/workspace/.mcp.json'
+        ),
+        (
+            '/home/coder/workspace/big.md',
+            'instruction_file',
+            '{}',
+            '5555555555555555555555555555555555555555555555555555555555555555',
+            99999::bigint,
+            'oversize',
+            'file exceeds 64KiB per-resource cap',
+            ''
+        )
+) AS v(source, body_kind, body, content_hash, size_bytes, status, error, source_path);