refactor(python): simplify plugin mechanics with standard idioms

[BjRo]

Why

The Python migrations left manual escaping, argument indexing, and resource cleanup that standard Python operations can express more directly.

What changed

• Use translation tables for Markdown, catalog, and TSV escaping; direct type guards for provider records; iterator/pair traversal for arguments; dataclasses.replace() for catalog rows; and Counter for report totals.
• Use contextlib.closing() for SQLite connections and remove a redundant catch-and-reraise without changing explicit transactions or cancellation propagation.
• Add the behavior-preservation invariant to docs/specs/python-quality.md and increment both plugin manifests for discovery, Git, review, decisions, adaptive delivery, and Langfuse observability.

Verification

• Python quality checks covered all 11 registered packages: 1,333 tests passed, with Ruff, strict mypy, and statement/branch coverage gates passing. Ran bun run check:python; after correcting one import-order lint issue, completed the remaining gates with bun run check:python --package plugins/capability/darrow-review/backend, bun run check:python --package plugins/orchestration/darrow-adaptive-delivery/backend, and bun run check:python --package plugins/orchestration/darrow-ticket-pipeline/backend.
• Passed bun run lint, bun run lint:ts, bun run lint:shell, bun run typecheck, bun run check:decisions, and bun run check:docs.
• Passed the Git, review, decisions, and adaptive-delivery backend/tests/fresh_install.py checks through their frozen runtime-only UV entrypoints. Passed discovery and Langfuse tests/fresh-install.test.sh with both bash and /bin/bash.
• Passed the packaged inspect-skill inspect checks for the seven affected skill entrypoints.
• Temporary differential probes matched five escaping functions across every Unicode code point and mixed escape sequences, 19,531 adaptive argument vectors, 781 frontier argument vectors, and malformed JSON/transcript cases.
• An independent fresh-context reader found no issues.

Review notes

Exact serialized bytes, literal flag-looking arguments, error precedence, and SQLite transaction behavior are the key compatibility constraints. Local validation ran on macOS with Python 3.13; native Linux/Windows and the other supported Python versions remain for CI. Live agent evals were not run because skill instructions and workflow behavior are unchanged. The larger typed-record redesign remains a separate follow-up.

Checklist

• I have read and followed CONTRIBUTING.md, including the contribution
  licensing terms.
• I added or updated the applicable invariant before implementation, or
  this change does not affect a capability invariant.
• I added or updated colocated evals, or this change does not affect skill
  behavior.
• I confirmed that each changed plugin remains self-contained, or this
  change does not affect plugin content.
• I ran bun run check:python, or this change does not affect registered
  Python packages or their repository quality infrastructure.