Expose Telegram slash commands via setMyCommands

[yaroslavyaroslav]

Summary

Expose the existing Telegram bot commands through Telegram's native slash-command menu and unify command definitions across Telegram, /help, and /api/commands.

Fixes #700

Changes

• register Telegram bot commands with setMyCommands when the Telegram adapter starts
• add a shared channel command catalog and reuse it for Telegram command registration, /help, and /api/commands
• include /agents and /agent in the shared command list
• update /agent so calling it without an argument returns usage guidance plus the live list of available agents from the server
• add default ChannelBridgeHandle methods for pending approvals and audio transcription so the current branch compiles with the existing bridge wiring

Testing

• cargo clippy --workspace --all-targets -- -D warnings passes
• cargo test --workspace passes
• Live integration tested (if applicable)

Additional verification:

• cargo check --workspace
• cargo audit
• cargo test -p openfang-channels test_handle_command_ -- --nocapture
• cargo build -p openfang-api --lib
• cargo build -p openfang-cli --bin openfang
• verified live daemon startup with real Telegram config
• verified GET /api/health
• verified GET /api/commands returns the unified command list, including /agents and /agent
• verified Telegram bot startup logs show webhook cleanup, polling mode activation, and bot command registration
• verified getMyCommands returns the registered Telegram command list

Note: cargo fmt --check currently fails on unrelated pre-existing formatting drift elsewhere in this checkout, outside this PR diff.

Security

• No new unsafe code
• No secrets or API keys in diff
• User input validated at boundaries

Telegram Bot API supports native command menus, but it does not support dynamic server-driven autocomplete for command arguments after /agent . This change uses the best available fallback by returning the live agent list when /agent is sent without an argument.