bug: manual /compact ignores user intent due to boundary hard-stop

[larock22]

Problem

Manual /compact currently calls force_compact(), but force mode still requires a valid retention boundary. If calculate_retention_boundary() returns 0, the command exits with:

Compaction skipped (no eligible boundary).

For users this is a hard-stop on an explicit command.

Why this is a UX bug

Auto-compaction can be conservative and boundary-safe.
Manual compaction should be user-authoritative (or provide an explicit override mode), not blocked by the same boundary gate.

Current behavior (proof points)

• src/tunacode/ui/commands/compact.py
  • calls controller.force_compact(...)
  • shows Compaction skipped (no eligible boundary). when outcome is non-compacted
• src/tunacode/core/compaction/controller.py
  • force_compact() delegates into _compact(...)
  • _compact(...) computes boundary via calculate_retention_boundary(...)
  • returns no_valid_boundary when boundary <= 0
• src/tunacode/core/compaction/summarizer.py
  • strict boundary validity rules (role/stop_reason/tool_result constraints)

Expected behavior

When the user explicitly runs /compact, command should not dead-end on no_valid_boundary.

Acceptable solutions:

1. Add manual override mode (/compact --unsafe or equivalent), or
2. Make /compact path use a fallback policy that still compacts with explicit warning semantics.

Acceptance criteria

• Repro session that currently returns no_valid_boundary no longer ends with dead skip on manual command.
• Auto-compaction safety policy remains unchanged for background/threshold triggers.
• Tests cover manual command outcome contract and boundary/override behavior.

[tunahorse]

Related UX research: #395 - /compact command has 6 UX issues with visual feedback