chat: rename the pi-mono provider router + purge residual pi names (Stage 4b/2)

Renames the AI-harness/provider settings router into the chat namespace and
clears the remaining "pi" identifiers from the chat stack.

- server-settings/pi-mono.ts → chat-providers.ts; piMonoRouter → chatProvidersRouter;
  route /server-settings/pi-mono → /server-settings/chat-providers (+ all callers)
- piId → providerId (PROVIDERS map + AIHarnessesSection UI), PiProvider → ChatProvider,
  PI_MONO_* query keys → CHAT_PROVIDERS_*, installPiMono → installAgent
- data-path: PI_CONFIG_DIR → AGENT_CONFIG_DIR (path ~/.pi/agent unchanged);
  drop dead getPiMonoDir/getPiMonoSessionDir exports
- settings: flip the vestigial defaultProvider literal 'pi' → 'chat' (never read;
  only defaultModel drives behavior); access-policy config key 'pi-access-policy'
  → 'chat-access-policy'
- misc: ModelSelector fallback label, TaskDefaults model grouping, CapabilityPage
  chat var, a stale stream-parser comment

Intentionally left (genuine external `pi`/opencode references, not ours to rename):
the `pi` binary install/version flow (@mariozechner/pi-coding-agent, `which pi`),
the ~/.pi/agent config path, PI_TOOLS_DIRS/PI_SEARXNG_URL runtime env-var contract,
TOOL.md `targets: pi` metadata, and the "Pi Mono" installer UI label.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/servers/sidecar/claude/stream-parser.ts

@@ -1,7 +1,7 @@
 /**
  * Claude Code NDJSON Stream Parser
  *
- * Parses the stream-json output format from Claude Code CLI into PiEvents.
+ * Parses the stream-json output format from Claude Code CLI into ChatEvents.
  * Handles: stream_event (deltas), assistant (text/tool_use), user (tool_result),
  * system init, and result messages.
  */