feat: Add context-safety-net plugin to mitigate auto-compact context loss

[jeshiomurmu]

Problem

Auto-compaction in long sessions frequently causes silent context degradation (e.g., issue #42542, #13112, #28721). Users lose track of critical "anchor" files, and the agent continues working blind. Currently, there is no deterministic, first-party way to recover this state without manually restarting the session or relying on brittle LLM summary parsing.

Solution

This PR introduces the context-safety-net plugin. It provides a deterministic, user-controlled safety net for compaction events.

Key Design Principles (Why this is safe to merge)

• Zero Core Changes: Built entirely on the existing PreCompact/PostCompact hook APIs and Statusline API. No modifications to the agent loop or compaction internals.
• Read-Only by Default: The PreCompact hook captures state using git diff and git ls-files. It does not use git stash or mutate repository state.
• Deterministic, Not Probabilistic: The PostCompact hook performs set arithmetic on file metadata. It does not parse natural language summaries.
• Explicit Agency: The plugin does not inject system messages or automatically reload files. It outputs a user-visible warning and exposes explicit slash commands (/snapshot, /restore, /compare) for recovery.
• Structured Config: Uses a schema-validatable .claude/context-anchors.json instead of plain text.
• Zero External Dependencies: Bash scripts use native grep/sed for JSON parsing, ensuring cross-platform compatibility (macOS, Linux, Windows Git Bash) without requiring users to install jq.

How it Works

• Before Compaction: The PreCompact hook snapshots the git state and anchor files to ~/.claude/context-safety-net/.
• After Compaction: The PostCompact hook checks if anchor files are missing from the working directory.
• User Notification: If missing, it prints a clean warning: ⚠️ Anchor files missing... Run /project:restore to reload.
• Recovery: The user runs /project:restore <name> to explicitly instruct Claude to re-read the anchor files into context.

Reviewer Notes

• Review surface: ~400 lines (Bash + Markdown).
• Risk: Near-zero. Fully opt-in, hooks are additive, no new APIs required.
• Issue Closure: Directly addresses the recovery mechanics requested in multiple compaction-related issues.

Checklist

• Plugin is opt-in and scoped to plugins/context-safety-net/
• Hooks use read-only git operations
• No system message injection or hidden context manipulation
• Anchor configuration is structured JSON