feat: agent reads context files and discovers skills locally (#23935)

Piggybacks on #23878. Moves instruction file reading and skill discovery
from `chatd` (server-side, via multiple `LS`/`ReadFile` round-trips
through the agent connection) to the agent itself (local filesystem
access).

This intentionally drops backward compatibility with older agents that
don't support the context-config endpoint. Agents and server are
deployed together; there is no rolling-update contract to maintain here.

## What changed

The agent's `GET /api/v0/context-config` response now returns
`[]ChatMessagePart` directly — the same types chatd persists. This
eliminates intermediate type conversions and makes the protocol
extensible.

| Field | Type | Description |
|---|---|---|
| `parts` | `[]ChatMessagePart` | Context-file and skill parts, ready to
persist |
| `working_dir` | `string` | Agent's resolved working directory |

Removed from the response: `instructions_dirs`, `instructions_file`,
`skills_dirs`, `skill_meta_file`, `mcp_config_files` — the agent reads
files locally and returns their content as parts.

Removed from chatd: all legacy `LS`/`ReadFile` fallback code
(`readHomeInstructionFile`, `readInstructionDirFile`, `DiscoverSkills`
via LS, etc).

## Why

The previous architecture had the agent resolve paths, serve them over
HTTP, then `chatd` make N+1 round-trips back through the agent
connection to read files. The agent has direct filesystem access and
should just read the files.

## Key design decisions

- **Agent returns `ChatMessagePart` directly** — same types chatd
persists. No intermediate `InstructionFileEntry`/`SkillEntry` types
needed.
- **`SkillMeta.MetaFile`** — persisted via `ContextFileSkillMetaFile` on
the skill part, so custom meta file names
(`CODER_AGENT_EXP_SKILL_META_FILE`) survive across chat turns.
- **No pre-read body** — `read_skill` always dials the workspace to
fetch the skill body on demand. Simpler than caching the body in the
response.
- **MCP config paths kept agent-internal** — `MCPConfigFiles()` getter,
not sent over the wire.
- **No backward compat fallback** — old agents that don't support
context-config get no instruction files. This is acceptable since agent
and server deploy together.

coderd/coderdtest/coderdtest.go

@@ -149,12 +149,13 @@ type Options struct {
 	OneTimePasscodeValidityPeriod time.Duration
 
 	// IncludeProvisionerDaemon when true means to start an in-memory provisionerD
-	IncludeProvisionerDaemon    bool
-	ProvisionerDaemonVersion    string
-	ProvisionerDaemonTags       map[string]string
-	MetricsCacheRefreshInterval time.Duration
-	AgentStatsRefreshInterval   time.Duration
-	DeploymentValues            *codersdk.DeploymentValues
+	IncludeProvisionerDaemon      bool
+	ChatdInstructionLookupTimeout time.Duration
+	ProvisionerDaemonVersion      string
+	ProvisionerDaemonTags         map[string]string
+	MetricsCacheRefreshInterval   time.Duration
+	AgentStatsRefreshInterval     time.Duration
+	DeploymentValues              *codersdk.DeploymentValues
 
 	// Set update check options to enable update check.
 	UpdateCheckOptions *updatecheck.Options
@@ -575,6 +576,7 @@ func NewOptions(t testing.TB, options *Options) (func(http.Handler), context.Can
 			// Force a long disconnection timeout to ensure
 			// agents are not marked as disconnected during slow tests.
 			AgentInactiveDisconnectTimeout: testutil.WaitShort,
+			ChatdInstructionLookupTimeout:  options.ChatdInstructionLookupTimeout,
 			AccessURL:                      accessURL,
 			AppHostname:                    options.AppHostname,
 			AppHostnameRegex:               appHostnameRegex,