Add bm-* slash commands for Basic Memory #2

Feature Description

Add plugin-owned in-session slash commands for Basic Memory in hermes-basic-memory, using a bm-* prefix to avoid collisions with Hermes core commands and other memory providers.

Motivation

Hermes already exposes Basic Memory to the agent through bm_* tools, but humans in CLI/gateway sessions do not have a lightweight slash-command surface for direct memory operations. Prefixing commands with bm-* keeps the UX discoverable while making ownership clear.

This should live in the hermes-basic-memory plugin rather than Hermes core because the commands are Basic Memory-specific and Hermes supports plugin-registered slash commands via ctx.register_command(...).

Proposed Commands

Initial command set:

• /bm-search <query> — search Basic Memory and return compact results.
• /bm-read <identifier> — read a note by title, permalink, or memory:// URL.
• /bm-context <identifier> — build/navigate Basic Memory context for a note or memory:// URL.
• /bm-recent [timeframe] — show recent Basic Memory activity/context.
• /bm-status — show plugin/provider status, mode, active project/path where available, and Basic Memory availability.

Optional follow-up command, depending on UX/safety:

• /bm-remember <text> — capture a quick note/memory. This should be designed carefully to avoid accidental writes or unclear folder/title behavior.

Implementation Notes

• Register commands from the plugin's register(ctx) entry point with ctx.register_command(name, handler, description=..., args_hint=...).
• Prefer command names like bm-search, bm-read, etc. so users invoke /bm-search, /bm-read, etc.
• Reuse the existing provider/MCP/tool path where practical instead of duplicating Basic Memory logic.
• Hermes core should not need changes unless plugin command metadata/autocomplete/dispatch proves insufficient.
• Include tests covering command registration and representative handler behavior.
• Update README/CHANGELOG with the command list and examples.

Acceptance Criteria

• When the Basic Memory plugin is enabled, Hermes exposes the new /bm-* slash commands in CLI/gateway sessions.
• Commands do not conflict with built-in Hermes slash commands.
• Search/read/context/status command handlers return clear, human-readable output.
• Recent activity behavior is documented, including any dependency on Basic Memory core support for updated-at based recent activity.
• Tests pass for plugin command registration and command behavior.