Goals HUD panel: sync completed goal eviction with viewport boundary

[hoblin]

What

When the Mneme boundary advances past every phantom pair belonging to a completed goal (set_goal / update_goal / finish_goal for that goal_id), the goal should disappear from the TUI HUD goals panel. Currently completed goals stay visible in the HUD until the session ends.

This is the goals counterpart to #431 — the same lifecycle gap that issue closed for sub-agents, applied to goals now that they live as phantom messages (#403) instead of in the system prompt.

Solution: flip evicted_at + reuse existing broadcast

The plumbing on the read path is already in place:

• Goal.not_evicted scope filters evicted_at IS NULL (app/models/goal.rb:31)
• Session#goals_summary already calls .not_evicted (app/models/session.rb:312)
• Goal#broadcast_goals_update fires goals_updated on every commit (app/models/goal.rb:136)

So the moment evicted_at is set on a goal, the after_commit broadcast pushes a fresh summary that omits it and the TUI HUD removes it. What's missing is the eviction hook that actually flips the column.

Goal traces in the viewport

A completed goal leaves multiple tool_call/tool_response pairs in the message stream:

1. set_goal — creation
2. update_goal — zero or more description edits
3. finish_goal — completion

A goal's HUD entry should drop only when none of these phantom pairs survive above the Mneme boundary. The lookup mirrors Session#subagent_trace_in_viewport? (app/models/session.rb:510) — query messages above mneme_boundary_message_id whose tool_name matches one of the goal tools and whose payload references this goal_id.

Eviction hook

In Mneme::Runner (lib/mneme/runner.rb:108), the existing refresh_subagent_visibility runs after each boundary advance. Add a sibling refresh_goal_visibility (called from the same place) that iterates session.goals.evictable (already-defined scope: completed.where(evicted_at: nil)) and, for each goal with no surviving traces, sets evicted_at = Time.current. The after_commit :broadcast_goals_update callback handles the TUI sync for free — no new event class or broadcaster needed.

What already works

• goals_updated broadcast on every Goal commit ✅
• goals_summary filters evicted goals ✅
• TUI handler handle_goals_updated consumes the broadcast (lib/tui/screens/chat.rb:472) ✅
• Goal.evictable scope (completed + not yet evicted) ✅

What's missing

• Session#goal_trace_in_viewport?(goal) — boundary-aware trace lookup ❌
• Mneme::Runner#refresh_goal_visibility — boundary-advance hook that flips evicted_at ❌

Why reuse goals_updated instead of a new goal_evicted event?

Sub-agents needed a dedicated SubagentEvicted event because the hud_visible column has no after_commit broadcast — child.update_column is a column-level write that bypasses callbacks. Goals already have a per-commit broadcast wired up, so a normal update!(evicted_at: Time.current) triggers the existing pipeline. Adding a parallel event would be redundant.

Related

• Move goals from system prompt to eviction-synced messages #403 — Goals to phantom messages (predecessor; deferred this lifecycle work)
• Sub-agent HUD panel: persist across TUI reconnects and sync with viewport eviction #431 — Sub-agent HUD eviction sync (the sibling pattern)
• Evicted goals stay visible in TUI #354 — Earlier fix that filtered not_evicted from goals_summary (still load-bearing)

References

• Sub-agent design: thoughts/shared/notes/2026-04-03/qa-brainstorm-event-loop-mailbox.md — "Sub-agent HUD Visibility Lifecycle" section
• Goal phantom-message design: thoughts/shared/notes/2026-04-02/goals-to-phantom-messages.md (LLM-side only — this issue covers the TUI gap left open there)