feat: add send_file built-in tool to send files from sandbox to Telegram

[chinkan]

Summary

• Adds a send_file built-in tool allowing the AI agent to send files from the sandbox to the Telegram user
• Follows the existing tool-based pattern — agent calls an explicit tool when it wants to send a file
• Sandbox path validation reused via validate_sandbox_path()
• File size checked before reading (50 MB Telegram limit)
• Friendly status message: "📤 Sending a file"
• Embed skills and agents at compile time via include_dir crate (no more CWD-relative path issues for systemd service)
• Remove two-layer SkillRegistry (instance + bundled with SkillSource) — simplified to single-layer
• Remove src/bin/setup.rs and setup.sh — use rustfox --setup instead
• Fix web wizard JS to only emit [skills]/[agents] sections when user provides non-empty values
• Fix /update-skills to backup modified files before overwriting (creates .bak)
• Remove bundled_directory from SkillsConfig/AgentsConfig — paths resolve under home by Config::resolve

What Changed

File	Change
src/tools.rs	Made validate_sandbox_path pub; added send_file ToolDefinition
src/agent.rs	Added send_file execution arm with Telegram send_document API call, ChatId plumbing; simplified SkillRegistry to single-layer; unified Available Agents section
src/platform/tool_notifier.rs	Added "📤 Sending a file" friendly name
src/skills/mod.rs	Removed SkillSource enum; simplified SkillRegistry to single skills HashMap; extracted shared preamble helper
src/skills/embed.rs	New — embedded skills/agents via include_dir! with seed + overwrite (backup) support
src/skills/loader.rs	Simplified to single-dir loading (no more SkillSource)
src/skills/seed.rs	Removed two-layer seed logic
src/skills/update.rs	Removed lock-file-based update logic
src/config.rs	Removed bundled_directory from SkillsConfig/AgentsConfig
src/main.rs	Replaced two-layer skill/agent loading with single-dir; uses embed::seed_* instead of seed::seed_dir_if_empty
src/platform/telegram.rs	/update-skills uses embed::overwrite_* with backup report
src/learning.rs	Updated load_skills_from_dir calls to single-dir signature
src/supervisor/classifier.rs	Updated SkillRegistry::register call (no SkillSource)
tests/supervisor_skill_packs.rs	Updated load_skills_from_dir call (no SkillSource)
src/setup/wizard.rs	Removed hardcoded [skills] directory = "skills"; updated default model to kimi-k2.6
src/setup/service.rs	Added {{RUSTFOX_PATH}} placeholder for systemd template
setup.sh	Removed — use rustfox --setup
src/bin/setup.rs	Removed — use rustfox --setup
skills-lock.json	Removed — replaced by embedded seed
Cargo.toml	Added include_dir dependency
config.example.toml	Removed bundled_directory references
scripts/services/rustfox.service.template	Added PATH environment variable
setup/index.html	Fixed to only emit [skills]/[agents] when user provides non-empty values
skills/news-fetcher/SKILL.md	Fixed stale invoke_subagent → invoke_agent references
skills/problem-solver/SKILL.md	Fixed stale invoke_subagent → invoke_agent references

Test Plan

• cargo build --release — builds clean
• cargo clippy -- -D warnings — no warnings
• cargo fmt --check — formatting clean
• cargo test — 378 tests pass