feat(compression): make protect_first_n configurable

The number of head messages preserved verbatim across context compactions
was previously hardcoded to 3 in AIAgent.__init__. Expose it as
`compression.protect_first_n` in config, matching the existing
`protect_last_n` pattern.

Motivation: users who rely on rolling compaction for long-running sessions
had the opening user/assistant exchange pinned as head forever, which
doesn't always match how they want the session framed after many
compactions. Lowering to 1 preserves the system prompt + first non-system
message; lowering to 0 preserves only the system prompt and lets the
entire first exchange age out naturally through the summary.

Semantics: `protect_first_n` counts non-system head messages protected
**in addition to** the system prompt, which is always implicitly protected
when present. Same meaning across both code paths:

  protect_first_n=0 → system prompt only (or nothing if no system message)
  protect_first_n=2 → system prompt + first 2 non-system messages (default)

This unifies the CLI path (which reads messages with the system prompt at
position 0) and the gateway path (where the gateway /compress handler
strips the system prompt before calling compress() — see
gateway/run.py L9150-9154 on the parent fork). Previously these two paths
disagreed:

  CLI path:     protect_first_n=1 → protect system prompt only
  Gateway path: protect_first_n=1 → protect first USER turn forever

In practice on long-running gateway sessions the old semantics pinned
whatever stale aside happened to be the first user message, reinserting
it into every compaction summary indefinitely.

Default chosen as 2 (not 3) so that the effective protected head count
remains 3 messages in the common case — assuming a system prompt is
present, default protection becomes system + 2 non-system = 3 total,
matching the pre-feature behaviour where `protect_first_n` was hardcoded
to protect 3 messages total. Sessions without a system prompt will see a
small behaviour change (2 protected head messages instead of 3), but this
is the rare path and the new semantics make the system-prompt-present
case the well-defined one.

Changes:

- agent/context_compressor.py: redefine protect_first_n as the count of
  non-system head messages protected beyond the implicit system-prompt
  guarantee; both paths converge. Constructor default updated to 2.
- hermes_cli/config.py: add `compression.protect_first_n` default (2),
  matching the new semantics. `show_config` label tweaked to
  'Protect first: N non-system head messages' for clarity.
- run_agent.py: read protect_first_n from config; 0 is now valid (system
  prompt is always implicitly protected).
- cli-config.yaml.example: document the new key and rationale.
- tests/agent/test_context_compressor.py: cover default, override, the
  end-to-end `protect_first_n=0` and `protect_first_n=1` behaviour,
  the no-system-prompt (gateway) path, and the new shared-semantics
  regression test.

Fixes #13751
Tested on Ubuntu 24.04.

cli-config.yaml.example

@@ -364,6 +364,18 @@ compression:
   # compression of older turns.
   protect_last_n: 20
 
+  # Number of non-system messages to protect at the head of the transcript, in
+  # ADDITION to the system prompt (which is always implicitly protected).
+  # Head messages are NEVER summarized — they survive every compression
+  # indefinitely. This gives stable early context for short/medium sessions,
+  # but in long-running sessions that rely on rolling compaction the pinned
+  # opening turns may not match how you want the session framed over time.
+  # Set to 0 to preserve ONLY the system prompt (plus the rolling summary
+  # and recent tail) — the cleanest configuration for long-running sessions.
+  # Default 2 preserves the system prompt plus the first user/assistant
+  # exchange (≈ 3 messages total when a system prompt is present).
+  protect_first_n: 2
+
   # To pin a specific model/provider for compression summaries, use the
   # auxiliary section below (auxiliary.compression.provider / model).