Make worktree prune preview-first and capacity warnings actionable

[chubes4]

Summary

High-volume workspace list still defaults to a 50-row page plus a compact capacity warning that only pointed at hygiene JSON, while raw Git already showed many prunable dead registrations. workspace worktree prune was immediately destructive, unbounded, and refreshed the full inventory.

This keeps inventory/cleanup bounded and fast, and makes the worktree-count warning preview-first:

• workspace worktree prune now defaults to a bounded dry-run of stale Git registrations (--limit=25, --until-budget=30s, --after-repo continuation). --yes applies. Live checkouts are never deleted.
• CLI/ability prune skips the full inventory refresh on preview and bounded apply.
• Worktree-count capacity advisories lead with workspace worktree prune --dry-run and inventory prune-missing --dry-run instead of only hygiene JSON.

Does not apply cleanup or delete worktrees.

Trackers

• Workspace stats and cleanup need a high-volume fast path #749
• workspace worktree cleanup: footguns + the conservatism/accumulation tension (must not hoard dead worktrees) #789

Verification

• Focused: php bin/dmc-test tests/worktree-prune-preview.php tests/worktree-prune-linked-primary.php tests/workspace-capacity-advisory.php tests/worktree-command-help-snapshots.php tests/workspace-list-cli-format-contract.php tests/workspace-show-cli-format-contract.php tests/workspace-compact-output.php tests/worktree-emergency-cleanup-lock-reactivation.php tests/workspace-command-startup-bounds.php
• Required suite: php bin/dmc-test — 148/150 passed. tests/worktree-add-lifecycle.php also fails on current origin/main (pre-existing). tests/workspace-command-startup-bounds.php passed in isolation and flaked once under the full suite.

Remaining gaps

• Default workspace list is still a 50-row page with cursor ceremony; --summary remains opt-in (workspace list should be bounded and summary-first on high-cardinality workspaces #1034 closed the bounded-page slice).
• Ghost missing_path inventory rows still need the separate inventory prune-missing preview/apply path.
• Dead worktree accumulation from live/active-no-signal/lifecycle-reconciliation rows is unchanged (workspace worktree cleanup: footguns + the conservatism/accumulation tension (must not hoard dead worktrees) #789, Cleanup should not rely on agents marking worktrees to find abandoned candidates #750).
• This PR does not apply prune or cleanup.

AI assistance

OpenAI gpt-5.6-sol via OpenCode general coding subagent inspected #749/#789, matching PRs, live inventory/hygiene, and raw Git prune candidates; implemented preview-first bounded prune and actionable capacity warnings; added deterministic tests; ran focused and repository tests; and opened this PR. Chris Huber owns direction and merge.