Add reversible opt-in budget guardrails and applied optimization fixes

[ekalb81]

Outcome

Allow users to enforce selected budgets and apply selected optimization remediations with preview, explicit consent, an audit journal, and reliable undo—without weakening Odometer's default read-only posture.

Security boundary

This issue introduces configuration writes and possibly agent hooks. Nothing in it may run merely because Odometer scanned a session or detected a finding. Implementation requires a focused security review before mutation paths are enabled.

Scope

• Define a small allowlisted action contract with target type, preconditions, preview, expected change, backup/restore plan, and postcondition verification.
• Support dry-run first; interactive apply requires per-action confirmation unless the user explicitly selects a bounded batch.
• Record a local audit journal with action version, redacted target identity, before/after hashes, result, and undo state.
• Implement undo using exact backups or validated inverse operations; refuse when the target changed externally.
• Add provider-specific budget guard adapters only where the agent supports documented hooks/statusline controls. Include a visible per-session override and emergency disable.
• Measure realized savings/outcomes through the workflow/correlation system rather than declaring an applied fix successful from configuration change alone.

DRY boundary

• Findings propose typed actions; they do not write files directly.
• One backend action executor owns validation, atomic writes, journaling, and undo.
• No generic shell-command action and no new broad Tauri filesystem/shell capability.

Acceptance criteria

• Preview is default and malformed/unknown action fields fail closed.
• Targets are resolved and revalidated immediately before mutation; symlink/path traversal and concurrent-change cases are covered.
• Declined elevation/confirmation changes nothing.
• Interrupted writes recover atomically and undo never overwrites unrelated user edits.
• Guard hooks expose exact install locations, active caps, overrides, and removal status.
• Neutral fixtures cover apply, batch, partial failure, rollback, external edits, stale findings, and full uninstall.

Dependency and ordering

• Phase: 3 — controlled expansion
• Priority: P1
• Depends on: Add opt-in live quota windows, budgets, pace projections, and alerts #43, Add workflow intelligence and a measurable optimization lifecycle #45
• Parent roadmap: Roadmap: close competitive analytics gaps on shared local-first foundations #36