docs: add CLAUDE.md and .agent/ guides for AI contributors

Add CLAUDE.md at the repository root to orient future Claude Code
instances, and split detailed constraints into .agent/:

- .agent/design.md    — architectural constraints (core small, duplication
  over abstraction, minimal changes, explicit over magical)
- .agent/security.md  — workspace/SSRF/shell sandbox boundaries
- .agent/gotchas.md   — config ${VAR}, Windows compat, templates,
  heartbeat virtual tool call, atomic writes, ruff format warning,
  skills extension point

Also updates .gitignore to not ignore .agent/.

.agent/design.md

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+# Design Constraints
+
+These rules govern architectural decisions. When adding a feature or fixing a bug, prefer paths that respect these boundaries.
+
+## Core stays small; extend at the edges
+
+New capabilities should be added via `channels/`, `tools/`, skills, or MCP servers. The files `agent/loop.py` and `agent/runner.py` form the critical core path; changes there should be minimal and justified. If a feature can live in a channel adapter, a tool, or an external MCP server, it should not be inlined into the agent loop.
+
+## Prefer duplication over premature abstraction
+
+Channels and providers are allowed to repeat similar logic (send retries, media handling, message splitting). Do not introduce complex base classes or shared helpers just to eliminate duplication across channel files. Each channel file should remain self-contained and readable on its own. The same applies to provider implementations.
+
+## Minimal change that solves the real problem
+
+Fix bugs by changing only what is necessary. Do not bundle unrelated refactors or clean-ups into a feature or bugfix PR. If a refactor is genuinely required, it should be a separate PR targeting `nightly`.
+
+## Explicit over magical
+
+Configuration must be declared explicitly in `config/schema.py` Pydantic models. Error handling should raise clear exceptions rather than silently correcting bad input. Provider auto-detection exists, but every resolution path must be traceable from the factory to the concrete provider class.

.agent/gotchas.md

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+# Common Gotchas
+
+## Do not use `ruff format`
+
+`CONTRIBUTING.md` mentions `ruff format`, but **do not run it** — it destroys git blame history. Only `ruff check` should be used.
+
+## Config `${VAR}` References
+
+`config/loader.py` resolves `${VAR}` patterns in `config.json` at load time. This is **not** a shell-like default-value syntax. If the environment variable is missing, `load_config` raises `ValueError` and the agent falls back to default configuration.
+
+Example valid usage:
+```json
+{ "providers": { "openrouter": { "apiKey": "${OPENROUTER_KEY}" } } }
+```
+
+## Windows Compatibility
+
+nanobot explicitly supports Windows. Key differences to keep in mind:
+- `ExecTool` uses `cmd /c` on Windows instead of `sh -c` (`shell.py`).
+- `cli/commands.py` forces `sys.stdout`/`stderr` to UTF-8 on startup to handle emoji and multilingual input.
+- MCP stdio server commands are normalized for Windows path separators (`mcp.py`).
+- Always use `pathlib.Path` for path manipulation; do not assume `/` separators.
+
+## Prompt Templates
+
+Agent system prompts and scenario-specific instructions live in `nanobot/templates/` as Jinja2 markdown files (`identity.md`, `platform_policy.md`, `HEARTBEAT.md`, `SOUL.md`, etc.). Changing these files alters agent behavior as directly as changing Python code. They are loaded by `utils/prompt_templates.py`.
+
+## Heartbeat Virtual Tool Call
+
+The heartbeat service (`heartbeat/service.py`) does not parse free-text LLM output. Instead, it injects a virtual `heartbeat` tool with `action: skip | run` into the conversation. Phase 1 is a structured decision; Phase 2 executes only on `run`. When adding new periodic background checks, follow this virtual-tool-call pattern rather than string matching.
+
+## Skills as Extension Point
+
+Built-in skills live in `nanobot/skills/` (markdown + YAML frontmatter format). Agent capabilities that are "know-how" rather than code should be added as skills, not hardcoded into the agent loop. External skills can be published to and installed from ClawHub.
+
+## Atomic Session Writes
+
+`agent/memory.py` writes `history.jsonl` atomically (temp file + fsync + rename + directory fsync). This guarantees durability across crashes. Do not replace this with a plain `open(..., "w")` write.

.agent/security.md

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+# Security Boundaries
+
+The agent operates with significant power (file system, shell, web). The following guards must not be bypassed when modifying related code.
+
+## Workspace Restriction
+
+Filesystem tools (`read_file`, `write_file`, `edit_file`, `list_dir`) resolve paths through `_resolve_path` (`agent/tools/filesystem.py`), which enforces that the resolved path must lie under `allowed_dir` (typically the configured workspace), plus the media upload directory (`get_media_dir()`) and any `extra_allowed_dirs`.
+
+Shell execution (`ExecTool`, `agent/tools/shell.py`) also respects `restrict_to_workspace`: if enabled and `working_dir` is outside the workspace, the command is rejected before execution.
+
+**Rule**: Any new path-handling logic must go through `_resolve_path` or perform an equivalent `allowed_dir` check.
+
+## SSRF Protection
+
+All outbound HTTP requests from agent tools must pass through `validate_url_target` (`security/network.py`). By default it blocks RFC1918 private addresses, link-local ranges, and cloud metadata endpoints (including `169.254.169.254`).
+
+The only escape hatch is `configure_ssrf_whitelist(cidrs)`, which reads from `config.tools.ssrf_whitelist` at load time.
+
+**Rule**: Do not add direct `httpx.get` / `requests.get` calls in tools. Route through the existing web fetch utilities or replicate the `validate_url_target` check.
+
+## Shell Sandbox
+
+`tools/sandbox.py` provides optional command wrapping. The only backend currently shipped is `bwrap` (bubblewrap), intended for containerized deployments. On Windows and bare-metal Linux without `bwrap`, commands run in the native shell with workspace restriction as the only guard.
+
+**Rule**: If adding a new sandbox backend, implement `_wrap_<name>(command, workspace, cwd) -> str` and register it in `_BACKENDS`.