fix(prune): separate file and context savings

[SUaDtL]

Summary

• Separate whole-file reduction from model-visible context savings with an explicit scope for every prune strategy.
• Preserve legacy file-metric aliases while correcting dry records, audit logs, state, CLI output, statusline output, and cold-cache nudge decisions across Claude, Codex, and Pi.
• Reassess the existing aggregate evidence and keep pruning plus its nudge off by default because no sampled row cleared the 80,000 context-token floor.

Decision

sidecar-collapse is file-only because it removes host bookkeeping. Strategies that edit model-visible message content are context-scoped. Existing transformation eligibility remains byte-for-byte behaviorally compatible, including Unicode edge cases.

Conflict level 2: extend the preview metric contract without changing prune transformations, defaults, or release versions. This PR is stacked on #313 because it updates the shared Pi support core introduced there.

Test plan

• wsl python3 -m unittest discover -s plugins/ca/hooks/tests -p test_*.py (1,022 passed, 1 skipped)
• All 42 .github/scripts/test_*.py suites
• Pi TypeScript typecheck and Vitest (544 passed, 1 skipped)
• Policy parity plus focused prune behavior (3 policy tests and 108 focused tests)
• Core sync, surface generation, host-package, plugin-reference, badge, license, JSON, secret-preview, and reproducible Pi build checks
• Independent security review: PASS
• Independent coverage review: CLEAN

[SUaDtL]

Hosted verification is green for exact head eff294caf89f4ee16533d5d1056bffc87e437ca8.

• CI: https://github.com/arbiterForge/codeArbiter/actions/runs/29736886144
• Docs: https://github.com/arbiterForge/codeArbiter/actions/runs/29736886149
• Ubuntu, macOS, and Windows hook and Pi adapter lanes passed for both supported Pi versions.
• Pi security analysis, payload gates, generated surfaces, reference graphs, manifests, docs, and aggregate merge readiness passed.

The PR base was temporarily changed to main and the PR was closed/reopened solely to trigger workflows that exclude stacked bases. It is restored to feat/pi-support, remains open, and is not merged.

[SUaDtL]

Superseded by #313, the single hackathon review surface. This PR's recorded head and changes are represented in #313's exact 16-PR / 29-commit / 126-path source manifest. PR #313 passed final exact-head CI at commit 6173b1d and remains open and unmerged. Closing this source PR to avoid parallel review; please continue review and discussion on #313.