feat(skill): add explicit project scope

[ousamabenyounes]

摘要 / Summary

根据维护者关闭 #390 时提出的方向，本 PR 实现了显式的 scope 参数：--scope=user|project。默认值仍为 user，不进行隐式推断。

Adds an explicit --scope=user|project parameter for skill registration. user
remains the default, and nothing is inferred from the working directory.

Addresses #333.

Design rationale

Issue #333 asked for project-local skill installation, and listed an explicit
--scope flag as one of its two options. #390 (@StevanusPangau) explored the
feature and identified the same target path, but derived the scope from the
working directory; it was closed in favour of designing an explicit scope
parameter. This PR implements that project-local target with an explicit flag
rather than inference, on the current installer.

Keeping user as the default is deliberate: an implicit write into the working
directory would contradict the workspace-isolation rules in docs/install.md
and the read-only-by-default install introduced in #577.

On agent-reach install, project scope is already covered by the --system
gate from #577 — install --scope=project without --system writes nothing.
Only agent-reach skill --install / --uninstall is ungated, and it is ungated
today for user scope too, so this PR did not add a second authorization flag
there. Instead the resolved absolute destination is printed before the first
write, so a project-scoped install is never silent about where it lands. Happy
to put the skill subcommand behind --system as well if you prefer.

Changes

• --scope=user|project on agent-reach install and
  agent-reach skill --install / --uninstall, defaulting to user
• project scope resolves <cwd>/.claude/skills/agent-reach through one helper
  shared by install and uninstall, which refuses to proceed when .claude or
  .claude/skills is a symlink
• the resolved absolute destination is printed before the first write
• skill --uninstall --scope=project removes that directory, so a
  project-scoped install can be undone by the CLI that created it;
  agent-reach uninstall stays user-scoped and never touches the working
  directory
• the workspace-policy wording in docs/install.md is kept byte-identical and
  the exception added as a separate line; agent_reach/skill/SKILL.md,
  SKILL_en.md and docs/update.md state the same rule and are updated in the
  same PR, so the agent-facing instructions do not tell an agent to refuse a
  supported flag
• CHANGELOG.md entry under ## [Unreleased] — happy to drop or re-target it
  if you keep the changelog on a different cadence

Documentation diffs are additions only: docs/install.md +19/-0,
docs/update.md +2/-0, SKILL.md +2/-0, SKILL_en.md +3/-0.

Test verification (RED → GREEN)

tests/test_skill_command.py from this branch applied to upstream/main with
no production changes (RED):

FAILED test_install_skill_rejects_unknown_scope
FAILED test_project_scope_announces_target_before_writing
FAILED test_project_scope_installs_only_in_current_project
FAILED test_project_scope_preserves_existing_skill_when_not_forced
FAILED test_project_scope_rejects_symlinked_skill_parent
FAILED test_project_scope_reports_expected_destination_copy_failure
FAILED test_project_scope_reports_expected_directory_creation_failure
FAILED test_project_scope_uninstall_rejects_symlinked_skill_parent
FAILED test_project_scope_uninstall_removes_only_project_skill
FAILED test_project_scope_uninstall_reports_removal_failure
FAILED test_project_scope_uninstall_reports_when_nothing_installed
FAILED test_project_scope_uninstall_unlinks_symlinked_target
FAILED test_skill_cli_accepts_project_scope
FAILED test_skill_cli_accepts_project_scope_uninstall
FAILED test_skill_command_forwards_explicit_scope
FAILED test_skill_command_forwards_uninstall_scope
FAILED test_system_install_forwards_project_scope
FAILED test_system_install_keeps_default_user_scope_call_compatible
FAILED test_uninstall_skill_rejects_unknown_scope
19 failed, 12 passed, 16 subtests passed

Patched branch (GREEN):

606 passed, 16 subtests passed

upstream/main baseline is 586 passed, measured in a clean worktree; this PR
adds 20 tests and removes none.

Each project-uninstall branch was mutation-checked rather than only covered —
breaking the scope guard, the symlinked-target unlink, the removal-failure
report or the not-found message each fails a test:

mutate: drop uninstall scope guard        1 failed, 30 passed
mutate: follow symlinked target           1 failed, 30 passed
mutate: swallow removal error             1 failed, 30 passed
mutate: drop not-found message            1 failed, 30 passed

Verification

• ruff check agent_reach tests — All checks passed
• mypy agent_reach — Success: no issues found in 35 source files
• pytest -q — 606 passed