Remove the dormant Phase 2 (scan + prune) prompts under pitch-only intake

[andrewsu]

Context

Both roles have moved to pitch-only intake (branch blackbird-prompt-refactor):

• A lab agent (pi_lab) posts only :bulb: pitches. A pitch @-mentions the hub and is auto-activated as a thread in Phase 3, so it never needs Phase 2 selection.
• BlackbirdBot (scout_hub) no longer scouts unsolicited results — an interview begins only from a pitch.

As a result, all four Phase 2 prompts are now permanent no-ops:

• prompts/roles/scout_hub/phase2-scan-filter.md — always returns selected_post_ids: []
• prompts/roles/scout_hub/phase2-prune.md — the interesting list never grows, so it never fires
• prompts/phase2-scan-filter.md / prompts/phase2-prune.md (the pi_lab globals) — same story

The LLM is still invoked for these phases every turn — there is no code guard skipping Phase 2 for these roles, so each agent spends an LLM call per turn producing an empty result.

Ask

Confirm we can skip Phase 2 entirely for pi_lab and scout_hub (a guard in the turn loop) and then delete the four dormant prompt files.

Verify before removing

• Phase 2 scan is the only writer to state.interesting_posts for these roles — nothing else populates it.
• Phase 5 no longer reads the interesting list (already removed for scout_hub; confirm pi_lab).
• Add the turn-loop guard to skip Phase 2 for these roles (prior note: "Skipping the phase for pi_lab saves one LLM call per PI per turn").
• Regenerate the golden-master prompt snapshots (test_agent_turn_gm, test_phase5_prompt_gm) to drop the Phase 2 assertions.

Docs

The Blackbird-facing docs/specs/2026-08-07-hub-bot-prompts.md has been streamlined to omit the dormant Phase 2 sections (they drive no behavior worth reviewing). docs/specs/2026-08-07-pi-bot-prompts.md has the same opportunity.

[ahueb]

Update: the pitch-only work went further than this issue tracked. Rather than deleting the dormant Phase-2 prompt files alone, the reconciliation branches removed Phase 2 entirely — the four prompt files and their doc reproductions (3977631, on PR #34's head), plus the engine builders, _phase2_scan_filter/_phase2_prune, the interesting_posts machinery, and the config (2e4b448 and follow-ups, in draft PR #35). The turn pipeline is now 1→3→4→5, with intake handled by the hub's automatic thread activation on every lab post.

Leaving this open for @andrewsu to close on review of #34/#35, since what shipped supersedes (rather than exactly implements) the tracked ask.

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