Unify CLI error handling on CliError and the three-line format

[JRS1986]

Summary

Completes the error-handling unification flagged in the codebase assessment (and designed but under-adopted since issue #92): CliError + the three-line error / next / see format is now the single failure surface for the CLI.

• main() catches CliError centrally and renders format_error(...) to stderr, returning exc.exit_code. The special-cased SystemExit dance around _normalize_args_tools_in_place is gone — normalize_tools' CliError just propagates.
• Domain validation errors raise CliError with recovery hints: session (git missing, not a repo, no active session, no start_commit, git command failures), memory (forbidden/restricted/unknown classes, secret-looking content, promote errors), pilot/doctor (unknown persona), skills (empty name), context (_validate_choice). Previously these were bare ValueError/RuntimeError printed as Error: ... by 7 copy-pasted handler catches — or, for skills/context, raw tracebacks.
• run_pilot no longer launders CliError into ValueError — the unknown-tool error now keeps its see: wiki link that the conversion was discarding.
• select-model missing-prompt now uses the same shape (exit code still 2).
• CliError drops frozen=True: Python's re-raise machinery (contextlib.__exit__) assigns __traceback__, which a frozen dataclass rejects with FrozenInstanceError. This was a latent landmine — any CliError crossing a @contextmanager crashed.
• Deliberately untouched: team.py and knowledge.py already catch their own raises at the public-function boundary and return warning-based results — converting their internal raises would have broken those catches.

Exit codes are unchanged throughout. Tests asserting RuntimeError/ValueError on the converted paths now assert CliError (same match strings); CHANGELOG documents the library-facing exception-type change under Unreleased.

Example (before → after)

$ coding-scaffold session diff
Error: No active session state file found.

$ coding-scaffold session diff
error: No active session state file found.
  next: Run `coding-scaffold session start` first.

Verification

• uv run pytest -q: 638 passed in 3.12s.
• uv run ruff check src tests: clean.
• Manual smoke test of session/memory/pilot/setup error paths — all render the three-line shape with correct exit codes.

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