fix: scope git askpass diff status updates to initiating chat (#22534)

## Problem

When the git askpass flow triggered diff status refreshes, it updated
**every chat** connected to the workspace. This was wasteful and could
cause confusing status updates on unrelated chats.

## Solution

Thread the chat ID through the entire git askpass flow so only the chat
that initiated the git operation gets updated:

1. **`coderd/chatd/chattool/execute.go`** — Sets `CODER_CHAT_ID` env var
on spawned processes (alongside the existing `CODER_CHAT_AGENT`)
2. **`cli/gitaskpass.go`** — Reads `CODER_CHAT_ID` from the environment
and sends it as a `chat_id` query parameter in the `ExternalAuthRequest`
3. **`codersdk/agentsdk/agentsdk.go`** — Adds `ChatID` field to
`ExternalAuthRequest` and encodes it as a query param
4. **`coderd/workspaceagents.go`** — Parses `chat_id` query param and
passes it through to `storeChatGitRef` and
`triggerWorkspaceChatDiffStatusRefresh`
5. **`coderd/chats.go`** — `storeChatGitRef` and
`refreshWorkspaceChatDiffStatuses` now scope updates to just the
initiating chat when a chat ID is provided, falling back to
all-workspace-chats behavior for backwards compatibility (non-chat git
operations)

cli/gitaskpass.go

@@ -78,6 +78,7 @@ func gitAskpass(agentAuth *AgentAuth) *serpent.Command {
 				Match:           host,
 				GitBranch:       gitBranch,
 				GitRemoteOrigin: gitRemoteOrigin,
+				ChatID:          inv.Environ.Get("CODER_CHAT_ID"),
 			})
 			if err != nil {
 				var apiError *codersdk.Error