docs: add git worktree sandbox guide, extend sandbox/plan-apply/context-tiers refs
- docs/modules/git-worktree-sandbox.md: new module guide for GitWorktreeSandbox
(PR #5998) covering execute/apply phases, non-git fallback, conflict handling,
and context hydration integration
- docs/reference/sandbox.md: add Strategy Selection section and Git Worktree
Sandbox section documenting execute/apply phases and non-git fallback
- docs/reference/plan_apply.md: add Git Worktree Merge-Based Apply section with
CLI output panels and fallback behaviour; update intro to reference PR #5998
- docs/reference/context_tiers.md: add Context Tier Hydration section documenting
ContextTierHydrator (PR #4219), hydration algorithm, exclusion rules,
configuration parameters, and LLMExecuteActor integration
- mkdocs.yml: add Git Worktree Sandbox to Modules nav
HAL9000
e7e7eedc4f