feat(gateway): add /context command for a detailed context-window view

A dedicated /context (alias /ctx) gateway slash command that gives a full
context-window view with:

- Usage gauge: visual bar + fraction + percentage + headroom
- Auto-compression threshold and how far away it is
- Compression count and how much the last one freed
- Cumulative session throughput (explicitly labelled as throughput,
  NOT context size — each call re-sends the window)
- Cascading fallback: running agent → cached agent → SessionStore metadata
  → rough transcript estimate

Not included (per current-main design):
- Cache reporting removed: commit 446b8e239 intentionally removed cache
  reporting from user-facing surfaces because providers that omit cached-token
  details produce misleading values
- Sync DB calls replaced with async_session_store (current main requires
  AsyncSessionStore with await)

Also rewords the /status tokens line from 'Cumulative API tokens (re-sent
each call)' to 'Lifetime tokens billed: ... (not your current context size;
use /context)' to reduce the recurring confusion that the cumulative figure
is the current context window.

Fixes salvation of PR #52184 (salvage commit replaces a 12K-commit-behind
fork branch with a fresh implementation against current main, incorporating
reviewer feedback from @whoislikemiha and the hermes-sweeper).

hermes_cli/commands.py

@@ -120,6 +120,8 @@ COMMAND_REGISTRY: list[CommandDef] = [
     CommandDef("status", "Show session, model, token, and context info", "Session"),
     CommandDef("egress", "Show Docker egress proxy status", "Session",
                args_hint="[status]", subcommands=("status",)),
+    CommandDef("context", "Show detailed context window view with usage gauge, compression stats, and throughput", "Session",
+               aliases=("ctx",)),
     CommandDef("whoami", "Show your slash command access (admin / user)", "Info"),
     CommandDef("profile", "Show active profile name and home directory", "Info"),
     CommandDef("sethome", "Set this chat as the home channel", "Session",