feat(cli): add Pi agent (pi-mcp-adapter) install support

[zh-xl-kang]

Summary

Add install/uninstall support for Pi agent (pi-mcp-adapter) to the CLI.

Relates to #649

Motivation

Pi agent (https://github.com/nicobailon/pi-mcp-adapter) is a coding agent that uses the standard mcpServers format in ~/.pi/agent/mcp.json. This PR adds detection, install, and uninstall support following the same pattern as Kiro/Cursor.

Changes

Detection (src/cli/cli.c, src/cli/cli.h)

• Add pi field to cbm_detected_agents_t struct
• Detect Pi agent by checking for ~/.pi/agent/ directory existence
• Add "Pi" to the detected agents output list

Install/Uninstall (src/cli/cli.c)

• Wire Pi into the editor-style install flow (alongside Kiro/Cursor/OpenClaw)
• Use cbm_install_editor_mcp() directly — no wrapper function needed
• Config path: ~/.pi/agent/mcp.json (standard mcpServers format)
• No instruction file (AGENTS.md) — Pi adapter does not read instruction files per its documentation
• Add uninstall support with cbm_remove_editor_mcp()
• Include Pi in the install plan JSON

Tests (tests/test_cli.c)

• Add cli_detect_agents_finds_pi: verifies detection requires ~/.pi/agent/ (not just ~/.pi/)
• Add ASSERT_FALSE(agents.pi) to cli_detect_agents_none_found
• All Pi-related tests pass

Documentation

• Update README.md: "11 agents" → "12 agents", add Pi config row
• Update docs/index.html (3 locations)
• Update docs/llms.txt
• Update pkg/npm/README.md

Design Decisions

1. Follow Kiro/Cursor pattern: Pi is placed in the editor agent group (not CLI agent group) because it uses the same cbm_install_editor_mcp() function directly, matching the established convention for agents that only need MCP config without instruction files.

2. No wrapper functions: Unlike Antigravity which has cbm_upsert_antigravity_mcp(), Pi uses cbm_install_editor_mcp() directly. This matches Kiro/Cursor and avoids unnecessary duplication.

3. No AGENTS.md: Per pi-mcp-adapter documentation, the adapter only reads MCP server configuration from JSON files and does not process instruction files. Verified by reviewing the adapter source.

4. Detection specificity: Detection checks for ~/.pi/agent/ (not just ~/.pi/) to avoid false positives, as tested in cli_detect_agents_finds_pi.

Testing

• All Pi-related tests pass:
  • cli_detect_agents_finds_pi ✓
  • cli_detect_agents_none_found ✓
• Code compiles cleanly with -Werror
• All commits are signed off (DCO compliant)

Files Changed

• src/cli/cli.h (+1 line)
• src/cli/cli.c (+17 lines)
• tests/test_cli.c (+24 lines)
• README.md (agent count + config table)
• docs/index.html (3 locations)
• docs/llms.txt
• pkg/npm/README.md

Total: +54 / -11 across 7 files