feat: add CLI commands for managing chat context from workspaces (#24105)

Adds `coder exp chat context add` and `coder exp chat context clear`
commands that run inside a workspace to manage chat context files via
the agent token.

`add` reads instruction and skill files from a directory (defaulting to
cwd) and inserts them as context-file messages into an active chat.
Multiple calls are additive — `instructionFromContextFiles` already
accumulates all context-file parts across messages.

`clear` soft-deletes all context-file messages, causing
`contextFileAgentID()` to return `!found` on the next turn, which
triggers `needsInstructionPersist=true` and re-fetches defaults from the
agent.

Both commands auto-detect the target chat via `CODER_CHAT_ID` (already
set by `agentproc` on chat-spawned processes), or fall back to
single-active-chat resolution for the agent. The `--chat` flag overrides
both.

Also adds sub-agent context inheritance: `createChildSubagentChat` now
copies parent context-file messages to child chats at spawn time, so
delegated sub-agents share the same instruction context without
independently re-fetching from the workspace agent.

<details><summary>Implementation details</summary>

**New files:**
- `cli/exp_chat.go` — CLI command tree under `coder exp chat context`

**Modified files:**
- `agent/agentcontextconfig/api.go` — `ConfigFromDir()` reads context
from an arbitrary directory without env vars
- `codersdk/agentsdk/agentsdk.go` — `AddChatContext`/`ClearChatContext`
SDK methods
- `coderd/workspaceagents.go` — POST/DELETE handlers on
`/workspaceagents/me/chat-context`
- `coderd/coderd.go` — Route registration
- `coderd/database/queries/chats.sql` — `GetActiveChatsByAgentID`,
`SoftDeleteContextFileMessages`
- `coderd/database/dbauthz/dbauthz.go` — RBAC implementations for new
queries
- `coderd/x/chatd/subagent.go` — `copyParentContextFiles` for sub-agent
inheritance
- `cli/root.go` — Register `chatCommand()` in `AGPLExperimental()`

**Auth pattern:** Uses `AgentAuth` (same as `coder external-auth`) —
agent token via `CODER_AGENT_TOKEN` + `CODER_AGENT_URL` env vars.

</details>

> 🤖 Generated by Coder Agents

---------

Co-authored-by: Michael Suchacz <203725896+ibetitsmike@users.noreply.github.com>

coderd/database/dbmock/dbmock.go

@@ -363,6 +363,20 @@ func (mr *MockStoreMockRecorder) CleanupDeletedMCPServerIDsFromChats(ctx any) *g
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "CleanupDeletedMCPServerIDsFromChats", reflect.TypeOf((*MockStore)(nil).CleanupDeletedMCPServerIDsFromChats), ctx)
 }
 
+// ClearChatMessageProviderResponseIDsByChatID mocks base method.
+func (m *MockStore) ClearChatMessageProviderResponseIDsByChatID(ctx context.Context, chatID uuid.UUID) error {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "ClearChatMessageProviderResponseIDsByChatID", ctx, chatID)
+	ret0, _ := ret[0].(error)
+	return ret0
+}
+
+// ClearChatMessageProviderResponseIDsByChatID indicates an expected call of ClearChatMessageProviderResponseIDsByChatID.
+func (mr *MockStoreMockRecorder) ClearChatMessageProviderResponseIDsByChatID(ctx, chatID any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "ClearChatMessageProviderResponseIDsByChatID", reflect.TypeOf((*MockStore)(nil).ClearChatMessageProviderResponseIDsByChatID), ctx, chatID)
+}
+
 // CountAIBridgeInterceptions mocks base method.
 func (m *MockStore) CountAIBridgeInterceptions(ctx context.Context, arg database.CountAIBridgeInterceptionsParams) (int64, error) {
 	m.ctrl.T.Helper()
@@ -1667,6 +1681,21 @@ func (mr *MockStoreMockRecorder) GetActiveAISeatCount(ctx any) *gomock.Call {
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetActiveAISeatCount", reflect.TypeOf((*MockStore)(nil).GetActiveAISeatCount), ctx)
 }
 
+// GetActiveChatsByAgentID mocks base method.
+func (m *MockStore) GetActiveChatsByAgentID(ctx context.Context, agentID uuid.UUID) ([]database.Chat, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetActiveChatsByAgentID", ctx, agentID)
+	ret0, _ := ret[0].([]database.Chat)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// GetActiveChatsByAgentID indicates an expected call of GetActiveChatsByAgentID.
+func (mr *MockStoreMockRecorder) GetActiveChatsByAgentID(ctx, agentID any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetActiveChatsByAgentID", reflect.TypeOf((*MockStore)(nil).GetActiveChatsByAgentID), ctx, agentID)
+}
+
 // GetActivePresetPrebuildSchedules mocks base method.
 func (m *MockStore) GetActivePresetPrebuildSchedules(ctx context.Context) ([]database.TemplateVersionPresetPrebuildSchedule, error) {
 	m.ctrl.T.Helper()
@@ -7780,6 +7809,20 @@ func (mr *MockStoreMockRecorder) SoftDeleteChatMessagesAfterID(ctx, arg any) *go
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "SoftDeleteChatMessagesAfterID", reflect.TypeOf((*MockStore)(nil).SoftDeleteChatMessagesAfterID), ctx, arg)
 }
 
+// SoftDeleteContextFileMessages mocks base method.
+func (m *MockStore) SoftDeleteContextFileMessages(ctx context.Context, chatID uuid.UUID) error {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "SoftDeleteContextFileMessages", ctx, chatID)
+	ret0, _ := ret[0].(error)
+	return ret0
+}
+
+// SoftDeleteContextFileMessages indicates an expected call of SoftDeleteContextFileMessages.
+func (mr *MockStoreMockRecorder) SoftDeleteContextFileMessages(ctx, chatID any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "SoftDeleteContextFileMessages", reflect.TypeOf((*MockStore)(nil).SoftDeleteContextFileMessages), ctx, chatID)
+}
+
 // TryAcquireLock mocks base method.
 func (m *MockStore) TryAcquireLock(ctx context.Context, pgTryAdvisoryXactLock int64) (bool, error) {
 	m.ctrl.T.Helper()