fix(worktree): use hard reset to correct file tree when branch base is wrong

[Tibsfox]

Summary

Fixes the worktree_branch_check mitigation that was using git reset --soft to correct wrong worktree base branches. The soft reset only moved the commit pointer — the working tree still contained main's files, causing silent data loss when the agent's commits overwrote feature branch code on merge-back.

• Changed --soft to --hard in all 4 workflow files that spawn worktree agents (execute-phase.md, quick.md, execute-plan.md, diagnose-issues.md). This is safe because the check runs as the very first action before any agent work — there are no uncommitted changes to lose.
• Removed the broken git rebase --onto attempt in execute-phase.md that could replay main's commits onto the feature branch if it succeeded.
• Added post-reset verification that aborts with exit 1 if the correction fails, preventing the agent from proceeding on a wrong branch base.
• Updated documentation from "Windows" to "all platforms" since the upstream EnterWorktree bug affects macOS, Linux, and Windows alike (as reported in Worktree isolation branches from main instead of current branch (macOS, not just Windows) #1957).
• Aligned settings.md worktree description to remove Windows-specific language, matching the workflow files.

Why --soft was wrong

git reset --soft moves HEAD but preserves the working tree and index. After the reset:

• Branch pointer → correct (feature branch HEAD) ✓
• Working tree → still main's files ✗
• Index → still main's files ✗

The agent then committed diffs against main's file state on top of the feature branch history. On merge-back, these commits overwrote the feature branch with main's code — exactly the data loss reported in #1957.

Why --hard is safe here

The worktree_branch_check runs as "FIRST ACTION before any other work" in a freshly-created worktree. At that point the agent has made zero uncommitted changes and done zero work. git reset --hard correctly resets the commit pointer, working tree, and index to the expected feature branch state.

Closes #1981
Related: #1957

Test plan

• All 2735 tests pass (1 pre-existing failure in stale-colon-refs.test.cjs unrelated to this change)
• Verified no remaining reset --soft.*EXPECTED_BASE references in the codebase
• Adversarial security review passed (0 critical/high/medium findings)
• Manual verification: run /gsd-execute-phase with workflow.use_worktrees: true on a feature branch diverged from main — confirm worktree agents operate on the correct file tree

🤖 Generated with Claude Code