fix(metrics): count opencode skill invocations

[bingran-you]

Problem

n_skill_invocations is 0 for every opencode rollout, including ones that
demonstrably loaded a skill. opencode's ACP skill call looks like this:

{"type": "tool_call", "kind": "other", "title": "skill", "status": "completed",
 "content": [{"type": "content", "content": {"type": "text",
   "text": "<skill_content name=\"polar-electrostatics-mentor\">\n# Skill: ..."}}]}

is_skill_invocation_event misses it four ways over:

1. kind is "other", not the canonical "skill".
2. The event carries no tool_name / name / function key, so
   _event_tool_name returns "".
3. The title check used an inline literal {"invoke_skill", "activate_skill"}
   which dropped the "skill" spelling that _SKILL_TOOL_NAMES — used by the
   kind check three lines above — already contains.
4. Content sniffing only recognized the OpenHands Tool: invoke_skill header.

This matters more than a cosmetic counter: n_skill_invocations is the field
that answers "did the agent actually use the skill?", and it is the control
variable for every with-skill vs no-skill comparison. Reading a hard 0 for an
entire harness makes those comparisons unfalsifiable — you cannot tell a skill
that didn't help from a skill that was never loaded.

Fix

Recognize both opencode signals, preserving the module's existing rule that a
tool declaring a real ACP kind is never reclassified from its title or output:

• <skill_content name="..."> counts only when it opens a structured
  tool-result text block (anchored \A, exactly as the OpenHands header is), so
  a grep whose output quotes the tag is not counted.
• Bare title == "skill" is honored only for unclassified kinds
  (""/other/tool) — the same gate content sniffing sits behind. The
  explicit invoke_skill / activate_skill spellings stay trusted outright, so
  existing behavior is unchanged. A read or execute tool titled skill
  (a filename, say) is still not a skill invocation.

Verification

Replayed against 206 recorded opencode trajectories from a 29-task,
6-arm evaluation:

	before	after
counter agrees with an independent <skill_content name=...> scan	0 / 206	206 / 206
with-skill rollouts reporting a nonzero count	0	56
no-skill rollouts falsely counted	0	0

The no-skill invariant that backstops the text-derived path still holds.

New tests cover the opencode envelope, a quoted-tag negative, and a
real-kind tool titled skill. tests/test_skill_invocation_artifacts.py
passes 10/10 (7 pre-existing unchanged); 697 tests pass across the
skill/metrics/trajectory suites. ruff check and ruff format --check clean.

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[chatgpt-codex-connector]

Name the guarded change in regression docstrings

The three newly added regression tests—this OpenCode-envelope case and the two negative cases below it—describe the behavior they protect but do not identify the PR or commit being guarded. Add the relevant PR/commit reference to each docstring so they comply with the repository's explicit regression-test convention.

AGENTS.md reference: AGENTS.md:L17-L17

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