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/coderd.go

@@ -168,6 +168,7 @@ type Options struct {
 	ConnectionLogger               connectionlog.ConnectionLogger
 	AgentConnectionUpdateFrequency time.Duration
 	AgentInactiveDisconnectTimeout time.Duration
+	ChatdInstructionLookupTimeout  time.Duration
 	AWSCertificates                awsidentity.Certificates
 	Authorizer                     rbac.Authorizer
 	AzureCertificates              x509.VerifyOptions
@@ -785,6 +786,7 @@ func New(options *Options) *API {
 			ProviderAPIKeys:                ChatProviderAPIKeysFromDeploymentValues(options.DeploymentValues),
 			AgentConn:                      api.agentProvider.AgentConn,
 			AgentInactiveDisconnectTimeout: api.AgentInactiveDisconnectTimeout,
+			InstructionLookupTimeout:       options.ChatdInstructionLookupTimeout,
 			CreateWorkspace:                api.chatCreateWorkspace,
 			StartWorkspace:                 api.chatStartWorkspace,
 			Pubsub:                         options.Pubsub,