feat(install): detect and configure pi coding agent

[Tensorboyalive]

Add pi (@earendil-works/pi-coding-agent) to the installer

install/uninstall auto-detect and configure 11 agents today, but not
pi. This adds pi as a
first-class target.

Why pi is wired differently (no mcpServers entry)

pi has no native MCP. Its README states: "No MCP. Build CLI tools with
READMEs (Skills), or build an extension that adds MCP support." So instead of an
MCP server entry, the pi adapter installs the three things pi actually consumes,
all driven by the existing CLI mode (codebase-memory-mcp cli <tool> '<json>'):

1. a pi extension → ~/.pi/agent/extensions/cbmem.ts — registers
   cbm_search_graph, cbm_trace_path, cbm_get_architecture,
   cbm_semantic_query, cbm_detect_changes, cbm_query_graph, cbm_index,
   cbm_list_projects as native pi tools that shell out to the binary;
2. the codebase-memory skill → ~/.config/agents/skills/codebase-memory/;
3. an AGENTS.md graph-first reminder, via the existing
   cbm_upsert_instructions sentinel upsert.

Changes

• cbm_detected_agents_t gains bool pi; cbm_detect_agents sets it.
• cbm_pi_config_dir honors $PI_CODING_AGENT_DIR, falls back to ~/.pi/agent
  (mirrors how cbm_claude_config_dir honors $CLAUDE_CONFIG_DIR).
• install_pi_config / uninstall_pi: write/remove the extension + skill +
  AGENTS.md block; respect --force and --dry-run; recorded in install --plan.
• README Multi-Agent Support table gains a pi row.
• Tests (tests/test_cli.c): detection (incl. $PI_CODING_AGENT_DIR),
  install writes extension/skill/AGENTS, uninstall reverses it.

Validation

• Clean build (-Wall -Wextra -Werror).
• install --plan → install → uninstall lifecycle verified against a fake
  HOME: extension/skill/AGENTS written then fully removed (sentinel gone).
• Memory handling matches the existing Codex/OpenCode upsert patterns
  (malloc/read_file_str/write_file_str, freed on every path).

No existing agent behavior is changed; this is purely additive.