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/querier.go

@@ -76,6 +76,7 @@ type sqlcQuerier interface {
 	CleanTailnetLostPeers(ctx context.Context) error
 	CleanTailnetTunnels(ctx context.Context) error
 	CleanupDeletedMCPServerIDsFromChats(ctx context.Context) error
+	ClearChatMessageProviderResponseIDsByChatID(ctx context.Context, chatID uuid.UUID) error
 	CountAIBridgeInterceptions(ctx context.Context, arg CountAIBridgeInterceptionsParams) (int64, error)
 	CountAIBridgeSessions(ctx context.Context, arg CountAIBridgeSessionsParams) (int64, error)
 	CountAuditLogs(ctx context.Context, arg CountAuditLogsParams) (int64, error)
@@ -215,6 +216,7 @@ type sqlcQuerier interface {
 	GetAPIKeysByUserID(ctx context.Context, arg GetAPIKeysByUserIDParams) ([]APIKey, error)
 	GetAPIKeysLastUsedAfter(ctx context.Context, lastUsed time.Time) ([]APIKey, error)
 	GetActiveAISeatCount(ctx context.Context) (int64, error)
+	GetActiveChatsByAgentID(ctx context.Context, agentID uuid.UUID) ([]Chat, error)
 	GetActivePresetPrebuildSchedules(ctx context.Context) ([]TemplateVersionPresetPrebuildSchedule, error)
 	GetActiveUserCount(ctx context.Context, includeSystem bool) (int64, error)
 	GetActiveWorkspaceBuildsByTemplateID(ctx context.Context, templateID uuid.UUID) ([]WorkspaceBuild, error)
@@ -893,6 +895,7 @@ type sqlcQuerier interface {
 	SelectUsageEventsForPublishing(ctx context.Context, now time.Time) ([]UsageEvent, error)
 	SoftDeleteChatMessageByID(ctx context.Context, id int64) error
 	SoftDeleteChatMessagesAfterID(ctx context.Context, arg SoftDeleteChatMessagesAfterIDParams) error
+	SoftDeleteContextFileMessages(ctx context.Context, chatID uuid.UUID) error
 	// Non blocking lock. Returns true if the lock was acquired, false otherwise.
 	//
 	// This must be called from within a transaction. The lock will be automatically