feat: persist chat instruction files as context-file message parts (#23592)

## Summary

Introduces a new `context-file` ChatMessagePart type for persisting
workspace instruction files (AGENTS.md) as durable, frontend-visible
message parts. This is the foundation for showing loaded context files
in the chat input's context indicator tooltip.

### Problem

Previously, instruction files were resolved transiently on every turn
via `resolveInstructions()` → `InsertSystem()` and injected into the
in-memory prompt without persistence. The frontend had no knowledge that
instruction files were loaded into context, and there was no way to
surface this information to users.

### Solution

Instruction files are now read **once** when a workspace is first
attached to a chat (matching how [openai/codex handles
it](https://developers.openai.com/codex/guides/agents-md)) and persisted
as `user`-role, `both`-visibility message parts with a new
`context-file` type. This ensures:

- **Durability**: survives page refresh (data is in the DB, returned by
`getChatMessages`)
- **Cache-friendly**: `user`-role avoids the system-message hoisting
that providers do, keeping the instruction content in a stable position
for prompt caching
- **Frontend-visible**: the frontend receives paths and truncation
status for future context indicator rendering
- **Extensible**: the same pattern works for Skills (future)

### Key changes

| Layer | Change |
|---|---|
| **SDK** (`codersdk/chats.go`) | Add `ChatMessagePartTypeContextFile`
with `context_file_path`, `context_file_content` (internal, stripped
from API), `context_file_truncated` fields |
| **Prompt expansion** (`chatprompt`) | Expand `context-file` parts to
`<workspace-context>` text blocks in `partsToMessageParts()` |
| **Chat engine** (`chatd.go`) | Add `persistInstructionFiles()`, called
on first turn with a workspace. Remove per-turn `resolveInstructions()`
+ `InsertSystem()` from `processChat()` and `ReloadMessages` |
| **Frontend** | Ignore `context-file` parts in `messageParsing.ts` and
`streamState.ts` (no rendering yet — follow-up will add tooltip display)
|

### How it works

1. On each turn, `processChat` checks if any loaded message contains
`context-file` parts
2. If not (first turn with a workspace), reads AGENTS.md files via the
workspace agent connection and persists them
3. For this first turn, also injects the instruction text into the
prompt (since messages were loaded before persistence)
4. On all subsequent turns, `ConvertMessagesWithFiles()` encounters the
persisted `context-file` parts and expands them into text automatically
— no extra resolution needed

codersdk/chats_test.go

@@ -184,6 +184,28 @@ func TestChatMessagePart_StripInternal(t *testing.T) {
 		assert.Equal(t, []byte("inline-data"), part.Data)
 	})
 
+	t.Run("StripsContextFileContent", func(t *testing.T) {
+		t.Parallel()
+		agentID := uuid.New()
+		part := codersdk.ChatMessagePart{
+			Type:                 codersdk.ChatMessagePartTypeContextFile,
+			ContextFilePath:      "/home/coder/AGENTS.md",
+			ContextFileContent:   "large content",
+			ContextFileAgentID:   uuid.NullUUID{UUID: agentID, Valid: true},
+			ContextFileOS:        "linux",
+			ContextFileDirectory: "/home/coder/project",
+		}
+		part.StripInternal()
+		// Internal fields stripped.
+		assert.Empty(t, part.ContextFileContent)
+		assert.Empty(t, part.ContextFileOS)
+		assert.Empty(t, part.ContextFileDirectory)
+		// Public fields preserved.
+		assert.Equal(t, "/home/coder/AGENTS.md", part.ContextFilePath)
+		assert.Equal(t, agentID, part.ContextFileAgentID.UUID)
+		assert.True(t, part.ContextFileAgentID.Valid)
+	})
+
 	t.Run("NoopOnCleanPart", func(t *testing.T) {
 		t.Parallel()
 		part := codersdk.ChatMessageText("hello")
@@ -209,12 +231,14 @@ func TestChatMessagePartVariantTags(t *testing.T) {
 	// If you add a new field to ChatMessagePart, either add a
 	// variants tag or add it here with a comment explaining why.
 	excludedFields := map[string]string{
-		"type":              "discriminant, added automatically by codegen",
-		"signature":         "added in #22290, never populated by any code path",
-		"result_delta":      "added in #22290, never populated by any code path",
-		"provider_metadata": "internal only, stripped by db2sdk before API responses",
+		"type":                   "discriminant, added automatically by codegen",
+		"signature":              "added in #22290, never populated by any code path",
+		"result_delta":           "added in #22290, never populated by any code path",
+		"provider_metadata":      "internal only, stripped by db2sdk before API responses",
+		"context_file_content":   "internal only, stripped before API responses (typescript:\"-\")",
+		"context_file_os":        "internal only, used during prompt expansion (typescript:\"-\")",
+		"context_file_directory": "internal only, used during prompt expansion (typescript:\"-\")",
 	}
-
 	knownTypes := make(map[codersdk.ChatMessagePartType]bool)
 	for _, pt := range codersdk.AllChatMessagePartTypes() {
 		knownTypes[pt] = true