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/workspaceagents_chat_context_internal_test.go

@@ -0,0 +1,128 @@
+package coderd
+
+import (
+	"context"
+	"database/sql"
+	"encoding/json"
+	"testing"
+	"time"
+
+	"github.com/google/uuid"
+	"github.com/sqlc-dev/pqtype"
+	"github.com/stretchr/testify/require"
+	"go.uber.org/mock/gomock"
+
+	"cdr.dev/slog/v3/sloggers/slogtest"
+	"github.com/coder/coder/v2/coderd/database"
+	"github.com/coder/coder/v2/coderd/database/dbauthz"
+	"github.com/coder/coder/v2/coderd/database/dbmock"
+	"github.com/coder/coder/v2/codersdk"
+)
+
+func TestUpdateAgentChatLastInjectedContextFromMessagesUsesMessageIDTieBreaker(t *testing.T) {
+	t.Parallel()
+
+	ctrl := gomock.NewController(t)
+	db := dbmock.NewMockStore(ctrl)
+	chatID := uuid.New()
+	createdAt := time.Date(2026, time.April, 9, 13, 0, 0, 0, time.UTC)
+	oldAgentID := uuid.New()
+	newAgentID := uuid.New()
+
+	oldContent, err := json.Marshal([]codersdk.ChatMessagePart{{
+		Type:               codersdk.ChatMessagePartTypeContextFile,
+		ContextFilePath:    "/old/AGENTS.md",
+		ContextFileContent: "old instructions",
+		ContextFileAgentID: uuid.NullUUID{UUID: oldAgentID, Valid: true},
+	}})
+	require.NoError(t, err)
+	newContent, err := json.Marshal([]codersdk.ChatMessagePart{{
+		Type:               codersdk.ChatMessagePartTypeContextFile,
+		ContextFilePath:    "/new/AGENTS.md",
+		ContextFileContent: "new instructions",
+		ContextFileAgentID: uuid.NullUUID{UUID: newAgentID, Valid: true},
+	}})
+	require.NoError(t, err)
+
+	db.EXPECT().GetChatMessagesByChatID(gomock.Any(), database.GetChatMessagesByChatIDParams{
+		ChatID:  chatID,
+		AfterID: 0,
+	}).Return([]database.ChatMessage{
+		{
+			ID:        2,
+			CreatedAt: createdAt,
+			Content: pqtype.NullRawMessage{
+				RawMessage: newContent,
+				Valid:      true,
+			},
+		},
+		{
+			ID:        1,
+			CreatedAt: createdAt,
+			Content: pqtype.NullRawMessage{
+				RawMessage: oldContent,
+				Valid:      true,
+			},
+		},
+	}, nil)
+
+	db.EXPECT().UpdateChatLastInjectedContext(gomock.Any(), gomock.Any()).DoAndReturn(
+		func(_ context.Context, arg database.UpdateChatLastInjectedContextParams) (database.Chat, error) {
+			require.Equal(t, chatID, arg.ID)
+			require.True(t, arg.LastInjectedContext.Valid)
+			var cached []codersdk.ChatMessagePart
+			require.NoError(t, json.Unmarshal(arg.LastInjectedContext.RawMessage, &cached))
+			require.Len(t, cached, 1)
+			require.Equal(t, "/new/AGENTS.md", cached[0].ContextFilePath)
+			require.Equal(t, uuid.NullUUID{UUID: newAgentID, Valid: true}, cached[0].ContextFileAgentID)
+			return database.Chat{}, nil
+		},
+	)
+
+	err = updateAgentChatLastInjectedContextFromMessages(
+		context.Background(),
+		slogtest.Make(t, &slogtest.Options{IgnoreErrors: true}),
+		db,
+		chatID,
+	)
+	require.NoError(t, err)
+}
+
+func insertAgentChatTestModelConfig(
+	ctx context.Context,
+	t testing.TB,
+	db database.Store,
+	userID uuid.UUID,
+) database.ChatModelConfig {
+	t.Helper()
+
+	sysCtx := dbauthz.AsSystemRestricted(ctx)
+	createdBy := uuid.NullUUID{UUID: userID, Valid: true}
+
+	_, err := db.InsertChatProvider(sysCtx, database.InsertChatProviderParams{
+		Provider:             "openai",
+		DisplayName:          "OpenAI",
+		APIKey:               "test-api-key",
+		ApiKeyKeyID:          sql.NullString{},
+		CreatedBy:            createdBy,
+		Enabled:              true,
+		CentralApiKeyEnabled: true,
+	})
+	require.NoError(t, err)
+
+	model, err := db.InsertChatModelConfig(sysCtx, database.InsertChatModelConfigParams{
+		Provider:             "openai",
+		Model:                "gpt-4o-mini",
+		DisplayName:          "Test Model",
+		CreatedBy:            createdBy,
+		UpdatedBy:            createdBy,
+		Enabled:              true,
+		IsDefault:            true,
+		ContextLimit:         128000,
+		CompressionThreshold: 70,
+		Options:              json.RawMessage(`{}`),
+	})
+	require.NoError(t, err)
+
+	return model
+}