refactor: decouple sessions and delegation

[lsongdev]

This refactor makes agent/subagent the same runtime concept: delegation is just a tool that invokes another normal profile-backed Agent with an isolated ephemeral session.

Session boundary

• add Agent.Run, which only advances the in-memory session and never writes it to disk
• move successful user-session persistence to NewRecordingSink, the boundary that already records session events
• keep an explicit save before user turns, and save failed ACP turns best-effort so partial state is not silently discarded
• delegated calls use the same session.Session type but never attach a recording/persistence sink
• keep RunAgentLoop as a one-line compatibility alias to Run

Delegation

• replace AgentRunner / SubagentTool with a plain DelegateFunc
• expose a single delegate(agent, task) tool
• delegate schema enumerates configured profiles and includes their optional descriptions
• return the delegated agent's final assistant response as the tool result
• propagate the caller context naturally and cap nested delegation depth at 4 to prevent accidental cycles

Tool composition

• remove built-in tool construction from Agent
• Manager assembles tools when it instantiates a profile-backed Agent
• add ProfileConfig.Tools; an empty list keeps the current default tool set, while a non-empty list selects exact tool names
• explicit built-in-only profiles do not initialize unrelated MCP servers
• MCP tools remain selectable by their existing mcp_<server>_<tool> names

Validation

A temporary PR-only CI step ran:

go test ./agent
go test ./tools -run '^TestDelegateToolCallsFunction$'
go test ./config -run '^$'

All passed, followed by the normal multi-platform make build. The temporary workflow change was then removed. After squashing the branch to one commit, the normal Release workflow ran again and passed (run #31).

The final branch is one commit over master; no workflow changes are included.