feat: add clineignore plugin

plugins/clineignore/README.md

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+# clineignore
+
+Blocks Cline tool calls that touch paths matched by a `.clineignore` file at
+the workspace root. This is the plugin successor to the built-in
+`.clineignore` support in the legacy VS Code extension.
+
+## Install
+
+```bash
+cline plugin install clineignore
+```
+
+Then create a `.clineignore` file at your workspace root using standard
+`.gitignore` syntax:
+
+```gitignore
+.env
+secrets/
+*.pem
+!include .ai-ignore-patterns
+```
+
+Lines starting with `!include <file>` pull in additional pattern files,
+resolved relative to the workspace root.
+
+## What it does
+
+- Skips `read_files`, `editor`, and `apply_patch` calls when any requested
+  path matches a `.clineignore` pattern. The model receives an error telling
+  it the path is blocked and to continue without it or ask you to update
+  `.clineignore`.
+- Skips `run_commands` calls when a file-reading command (`cat`, `less`,
+  `more`, `head`, `tail`, `grep`, `awk`, `sed`, and the PowerShell
+  equivalents) targets an ignored path. Pipelines and `&&`/`;` sequences are
+  scanned per segment.
+- Adds a system prompt rule (when `.clineignore` exists at session start) so
+  the model knows blocked paths are intentional and how to proceed.
+- Re-reads `.clineignore` when its modification time changes, so edits take
+  effect mid session without a restart.
+- The `.clineignore` file itself is always treated as ignored, matching the
+  legacy behavior.
+
+## What it does not do (security notes)
+
+This plugin is context hygiene, not a security boundary. It reduces the
+chance of ignored files ending up in model context, but a determined agent
+can still reach them:
+
+- Shell commands other than the recognized file readers are not scanned
+  (redirection, subshells, `xargs`, scripts, and so on).
+- Command scanning uses naive whitespace tokenization with no quote
+  awareness.
+- `search_codebase` results are not filtered, so matches inside ignored
+  files can still appear in search output.
+- Paths outside the workspace root are always allowed.
+
+Do not rely on it to protect real secrets from the model or the provider.
+Keep secrets out of the workspace, or accept that they may be read.
+
+## Requirements
+
+- A Cline surface that runs SDK plugins (CLI, SDK, Kanban).
+- No external services. The `ignore` npm package is the only dependency.
+
+## Example
+
+```bash
+echo ".env" > .clineignore
+cline -i "Read .env and tell me what is in it"
+# The read_files call is skipped with a .clineignore error instead.
+```