feat(pst): add Rule 25 context hygiene and output compression

[pstaylor-patrick]

Summary

• Adds Rule 25 to SKILL.md: context hygiene and output compression, covering pre-execution bash output limiting, content-type routing, and a ~50-line skip threshold. Numbered 25 (Rule 24 is already Best-of-N tournament).
• Amends Stage 0.5 in Rule 19 to apply Rule 25 heuristics when assembling the Opus planner context bundle.
• Adds a Rule 25 section to REFERENCE.md with: pre-execution limiting examples (table), content-type routing table, CCR pattern grounded in the session scratchpad, accurate Anthropic prefix-cache note, and verbosity-at-tail drafting heuristic.

Inspired by headroom. Adversarial-reviewed; five findings addressed (removed inapplicable recency clause, replaced 250-token threshold with ~50-lines, corrected prefix-cache claim, grounded CCR in Read tool + scratchpad, reframed verbosity-at-tail as a heuristic).

Test plan

• /pst loads without errors; Rule 25 appears in doctrine
• Stage 0.5 description in Rule 19 references Rule 25
• REFERENCE.md Rule 25 section renders correctly with tables
• No em dashes in added content (scripts/pst-emdash.rb check)

🤖 Generated with Claude Code

[pstaylor-patrick]

pst:ready attestation -- tournament winner: Strategy A (15/15)

Auto-validated test plan items:

• Stage 0.5 description in Rule 19 references Rule 25 (grep confirmed)
• REFERENCE.md Rule 25 section renders correctly with tables (section + table header confirmed)
• No em dashes in added content (0 matches for U+2014)
• /pst loads without errors; Rule 25 appears in doctrine (manual)

CI: lint pass. Open threads: 0. Ready to merge.