Add session_takeover tool for taking over recalled sessions in place

[luckeyfaraday]

Problem

agent_takeover can only hand over sessions this Athena Code process spawned — it's gated on an in-memory handle in the agents map. A session found via session_recall (or /find-sessions) has no handle, so the model had no in-place takeover tool for historical sessions and fell back to spawning a new terminal window. The in-place machinery (execResume / resumeCommand / the athena.agent.takeover event the TUI already listens for) was the whole time one event-emit away — it takes a plain { agent, sessionId, workspace }, not a live handle.

What this adds

A sibling tool, session_takeover, that bridges recall → the existing takeover event for any indexed session, no live handle required. No TUI changes — athena-takeover.tsx already handles the event for any resumable session.

• session/agent/session-takeover.ts (new) — pure, side-effect-free planSessionTakeover(): validates the recalled { agent, session_id } against the cross-agent index, resolves the workspace, and builds either an in-app handover or a visible-terminal resume.
• sessionindex.ts — indexedSessionWorkspace(): a stale id from recall now produces a clean not-found error instead of a silently dropped handover, and the session's own indexed workspace is used so it resumes in its own repo (better than defaulting to the current worktree for cross-workspace sessions).
• Supports all five agents including athena (resumed through process.execPath, which localAgentResumeCommand omits because athena isn't a spawnable subprocess agent).
• The tool() wrapper in tool/agent-local.ts only emits / launches; registered as session_takeover in the overlay plugin tool map (plugin/athena.ts).

Design notes

• No patch edit needed. Tool registration lives in the overlay's plugin/athena.ts (a full overlay file), not in a patch hunk — the patch only wires AthenaPlugin in as a builtin. patch-apply/patch-drift are unaffected.
• Testability: the repo's tests target dependency-free lib files (the tool() wrappers import @opencode-ai/plugin/GlobalBus, absent standalone). The logic is therefore in the pure planner, whose in_app plan fields are the event payload verbatim; the wrapper is structurally identical to the proven AgentTakeoverTool for the emit/terminal mechanics.
• No-TUI failure mode: kept the cheap "if nothing happens, run /find-sessions" guidance, matching agent_takeover (no ack handshake).

Tests

Added 4 cases to test/agent-local.test.ts (in-app payload, terminal mode for athena + claude, explicit workspace override, rejection of unknown agents / stale ids).

bun test test  →  98 pass, 0 fail

🤖 Generated with Claude Code