feat(skills): add backtester-optimization session skill

[knowttl]

Lands the backtester-optimization skill authored by the nextrade optimization session, so /backtester-optimization is invocable in future firstmate sessions.

Docs-only addition of one file: .agents/skills/backtester-optimization/SKILL.md.

• Frontmatter matches captain-invocable siblings (codev-session, firstmate-sync): user-invocable: true plus metadata.internal: true.
• Session-transient pointers cleaned: the worked example (pooled 9-asset -7.6R / PF 0.96 to +49.2R / PF 1.72) is kept as an illustrative example, but the dated session reference and its specific data-dir path are reframed generically.
• Cross-references verified: .agents/skills/codev-session/ and .agents/skills/decision-hold-lifecycle/ both exist; decision-hold-lifecycle is now named as a skill in the text.
• AGENTS.md intentionally untouched: section 13 lists agent-only reference skills, and no captain-invocable sibling (codev-session, bearings, ahoy, firstmate-sync) is listed there. Adding this one would be inconsistent and would grow the always-loaded file.
• bin/fm-doc-audience-check.sh passes (70 surfaces, 259 local links). No no-mistakes run, per the direct-PR delivery mode.

[knowttl]

Closing as a duplicate/misplacement: the backtester-optimization skill is nextrade-specific and belongs in the nextrade repo (tracked as nextrade-5opb), not firstmate's shared surface. Firstmate stays aware of the option and dispatches a nextrade crew to invoke it.