feat: load browse/ docs on demand - not on every /gstack invocation

**Problem**

\`gstack/SKILL.md\` inlines the full \`browse/\` documentation (~360 lines: command reference, QA workflows, tips) via \`gen:skill-docs\`. Every \`/gstack\` invocation loads all of it into context, including sessions where \`\$B\` is never called.

**Proposed fix**

Move the browse content to \`browse/SKILL.md\` and source it conditionally in the preamble when \`\$B\` is first invoked. Same pattern as the rest of gstack's skill structure.

**Why**

The browse reference earns its context budget in a QA session. In a \`/review\` or \`/ship\` session it adds ~360 lines of overhead with no payoff. Loading it every time goes against the "Search Before Building" principle already in gstack.