…efault

Address PR #484 review feedback:

* item 1 — plumb the gate to ``generate_all_settings`` so every front-end
  goes through it. The CLI was already gated; the MCP tools
  (``mem_context_sync`` / ``mem_context_generate``) and the Web route
  (``POST /settings-sync`` aliased as ``/context/settings/sync``) were
  bypassing the confirmation and silently writing
  ``~/.claude/settings.json``. Now the lib refuses any target whose
  resolved path is outside the resolved project root, marking those
  generators ``status="needs_confirmation"`` instead. Front-ends pass
  ``allow_host_writes=True`` after acknowledgement (CLI ``--yes`` /
  confirm, MCP tool param, Web request body). Project-scope generators
  are unaffected.

* item 2 — generic ``--yes`` help text ("before writing settings files
  outside this project") so it does not rot the moment a non-Claude
  generator is added.

* item 3 — CHANGELOG ``[Unreleased]`` ``Added`` entry covers the new
  prompt, the ``--yes`` flag, the lib gate, the front-end coverage,
  and the symlink resolution.

* item 4 — ``Path.resolve(strict=False)`` on both target and project
  root so a symlinked ``<project>/.claude → ~/.claude`` cannot smuggle
  a host write past the gate. Strict=False keeps the call cheap when
  the target file does not exist yet.

  (Item 5 — preview of the diff before the prompt — left as a
  follow-up; out of scope here.)

API change for libraries that call ``generate_all_settings`` directly:
the function now takes a keyword-only ``allow_host_writes: bool = False``
parameter. The default is the safe one — old callers that imported the
function without this kwarg now get a ``needs_confirmation`` status for
host-scope generators instead of a silent write. Also exports the new
helper ``host_write_targets(project_root)`` so a UI can list pending
paths before posting back with ``allow_host_writes=True``.

Tests:
* Library — 9 new cases in ``TestGenerateAllSettingsHostWriteGate``:
  default refuses, ``allow_host_writes=True`` proceeds,
  ``diff_settings`` unaffected (still read-only),
  no-runtime + no-canonical still skip (not needs_confirmation),
  ``host_write_targets`` lists / empties as expected, and a symlink
  case that confirms ``Path.resolve()`` follows the link.
* MCP — new ``test_server_tools_context_settings_gate.py`` (4 cases)
  pins the ``allow_host_writes`` threading through both
  ``mem_context_sync`` and ``mem_context_generate``.
* Web — 2 new cases in ``TestSettingsSync`` covering the POST default
  refusal and the ``{"allow_host_writes": true}`` proceed path.

Full suite: 2509 passed locally. ``ruff check`` + ``ruff format
--check`` + ``mypy`` on the four touched source files all clean.

Co-Authored-By: Claude <[email protected]>