Inspired by Claude Code: /compress here [N] — boundary-aware 'summarize up to here' (#35048)

Adds a user-chosen compression boundary to the existing /compress command.
/compress here [N] summarizes everything except the most recent N exchanges
(default 2), which are preserved verbatim — letting the user pick the
compression boundary instead of relying on the automatic token-budget heuristic.

Inspired by Claude Code's Rewind 'Summarize up to here' action (v2.1.139,
Week 20, May 2026): https://code.claude.com/docs/en/whats-new/2026-w20

- hermes_cli/partial_compress.py: pure split/parse helpers + seam-alternation
  guard (shared by CLI and gateway).
- cli.py / gateway/run.py: route 'here [N]' / '--keep N' to partial compression;
  compress only the head, re-append the verbatim tail through the seam guard.
- Preserves message-flow role alternation (seam guard merges any illegal
  user->user / assistant->assistant adjacency).
- Reuses the existing _compress_context session-rotation/lock machinery — no
  changes to the compression core.
- Bare /compress (full) and /compress <focus> behavior unchanged.

Tests: 12 helper unit tests + 5 CLI integration tests + E2E (interleaved
tool-call transcript, degenerate/multimodal seams, real handler path).

website/docs/reference/slash-commands.md

@@ -43,7 +43,7 @@ Type `/` in the CLI to open the autocomplete menu. Built-in commands are case-in
 | `/retry` | Retry the last message (resend to agent) |
 | `/undo` | Remove the last user/assistant exchange |
 | `/title` | Set a title for the current session (usage: /title My Session Name) |
-| `/compress [focus topic]` | Manually compress conversation context (flush memories + summarize). Optional focus topic narrows what the summary preserves. |
+| `/compress [here [N] \| focus topic]` | Manually compress conversation context (flush memories + summarize). `/compress here [N]` summarizes everything except the most recent N exchanges (default 2), kept verbatim — pick your own compression boundary. A focus topic narrows what a full summary preserves. |
 | `/rollback` | List or restore filesystem checkpoints (usage: /rollback [number]) |
 | `/snapshot [create\|restore <id>\|prune]` (alias: `/snap`) | Create or restore state snapshots of Hermes config/state. `create [label]` saves a snapshot, `restore <id>` reverts to it, `prune [N]` removes old snapshots, or list all with no args. |
 | `/stop` | Kill all running background processes |
@@ -206,7 +206,7 @@ The messaging gateway supports the following built-in commands inside Telegram,
 | `/retry` | Retry the last message. |
 | `/undo` | Remove the last exchange. |
 | `/sethome` (alias: `/set-home`) | Mark the current chat as the platform home channel for deliveries. |
-| `/compress [focus topic]` | Manually compress conversation context. Optional focus topic narrows what the summary preserves. |
+| `/compress [here [N] \| focus topic]` | Manually compress conversation context. `/compress here [N]` keeps the most recent N exchanges (default 2) verbatim and summarizes the rest. A focus topic narrows what a full summary preserves. |
 | `/topic [off\|help\|session-id]` | **Telegram DM only.** Manage user-managed multi-session topic mode. `/topic` enables it or shows status; `/topic off` disables it and clears bindings; `/topic help` shows usage; `/topic <session-id>` inside a topic restores a previous session. See [Multi-session DM mode](/user-guide/messaging/telegram#multi-session-dm-mode-topic). |
 | `/title [name]` | Set or show the session title. |
 | `/resume [name]` | Resume a previously named session. |