feat(subagents): add subagents system

[brianjlandau]

Summary

Adds a full subagents system to Crush — discoverable, configurable child agents that the coordinator can dispatch via the agent tool.

• Subagent definitions — Markdown files with YAML frontmatter (name, description, model, provider, effort, color, permissionMode, tools, disallowedTools, skills, mcp_servers) discovered from configurable paths
• Dispatcher tool — dynamic subagent_type enum built from active agents; builds each subagent with its own model/prompt/memory/permissions at dispatch time
• Skills — pinned skills: frontmatter suppresses full skill discovery; absent skills: keeps full access
• Color — eight named palette values + auto (deterministic hash); used in running-status panel and modal
• Effort — low/medium/high translated to provider-specific thinking budgets
• Permission modes — default and bypassPermissions (auto-approves the sub-session)
• Runtime tracker — tracks in-flight subagent sessions; feeds the inline status panel and modal
• TUI — inline running-subagents panel in the sidebar; modal (Ctrl+X) with Running + Library tabs; Library enable/disable toggle; parent breadcrumb + Ctrl+Up navigation; ctrl+x in help bar
• Model cache — resolveModelByID results memoized per (modelID, provider) and cleared on config reload
• Robustness — build/memory/prompt failures surfaced as tool-error responses (not turn aborts); all Validate overlaps reported

Test plan

• Build: CGO_ENABLED=0 GOEXPERIMENT=greenteagc go build ./...
• Tests: CGO_ENABLED=0 GOEXPERIMENT=greenteagc go test ./internal/subagents/... ./internal/agent/...
• Drop a .md subagent file in the discovery path — verify it appears in Library tab (Ctrl+X); also test @-mention completion in the editor
• Dispatch the subagent by name — verify Running tab shows it with correct color/model
• Disable it in Library (space) — verify it disappears from the agent tool enum
• Subagent with invalid model: — verify parent agent gets a readable tool-error, not a crash
• Press Ctrl+X → check help bar shows ctrl+x subagents; press Ctrl+G → binding in full help

🤖 Generated with Claude Code

[charmcli]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[brianjlandau]

I have read the Contributor License Agreement (CLA) and hereby sign the CLA.

[Zim-Inn]

why subagents? is it useful for coding?