fix(prompts): make skill loading an active directive in auto-mode units

The execute-task, plan-slice, and research-slice prompts all include a
passive instruction to 'use GSD Skill Preferences to decide which skills
to load.' In practice, auto-mode agents never act on this — across 30+
execution units in a real milestone, zero skill files were read.

The root cause is that the passive wording ('use it to decide') gets
overridden by the stronger 'don't re-research, just build what the plan
says' directive in execute-task. The agent treats skill loading as
optional and skips it 100% of the time.

This change rewrites the skill instruction in all three prompts from
passive guidance to an explicit action:
- execute-task: 'read its SKILL.md file now — before writing any code'
- plan-slice: 'read any skill files relevant to this slice's technology
  stack before decomposing'
- research-slice: 'read any skill files relevant to this slice's
  technology stack before exploring code'

The execute-task change also points agents to both the GSD Skill
Preferences block AND the <available_skills> catalog, since both are
present in the system prompt but the old instruction only referenced
the preferences block.

The plan-slice change adds guidance to note relevant skills in task
plans, so executors know which skills to load without rediscovering
them.

src/resources/extensions/gsd/prompts/execute-task.md

@@ -26,7 +26,7 @@ A researcher explored the codebase and a planner decomposed the work — you are
 
 Then:
 0. Narrate step transitions, key implementation decisions, and verification outcomes as you work. Keep it terse — one line between tool-call clusters, not between every call.
-1. If a `GSD Skill Preferences` block is present in system context, use it to decide which skills to load and follow during execution, without relaxing required verification or artifact rules
+1. **Load relevant skills before writing code.** Check the `GSD Skill Preferences` block in system context and the `<available_skills>` catalog in your system prompt. For each skill that matches this task's technology stack (e.g., React, Next.js, accessibility, component design), `read` its SKILL.md file now. Skills contain implementation rules and patterns that should guide your code. If no skills match this task, skip this step.
 2. Execute the steps in the inlined task plan
 3. Build the real thing. If the task plan says "create login endpoint", build an endpoint that actually authenticates against a real store, not one that returns a hardcoded success response. If the task plan says "create dashboard page", build a page that renders real data from the API, not a component with hardcoded props. Stubs and mocks are for tests, not for the shipped feature.
 4. Write or update tests as part of execution — tests are verification, not an afterthought. If the slice plan defines test files in its Verification section and this is the first task, create them (they should initially fail).

src/resources/extensions/gsd/prompts/plan-slice.md

@@ -33,7 +33,7 @@ Then:
 1. Read the templates:
    - `~/.gsd/agent/extensions/gsd/templates/plan.md`
    - `~/.gsd/agent/extensions/gsd/templates/task-plan.md`
-2. If a `GSD Skill Preferences` block is present in system context, use it to decide which skills to load and follow during planning, without overriding required plan formatting
+2. **Load relevant skills.** Check the `GSD Skill Preferences` block in system context and the `<available_skills>` catalog in your system prompt. `read` any skill files relevant to this slice's technology stack before decomposing. When writing task plans, note which installed skills are relevant in the task description so executors know which to load.
 3. Define slice-level verification — the objective stopping condition for this slice:
    - For non-trivial slices: plan actual test files with real assertions. Name the files.
    - For simple slices: executable commands or script assertions are fine.

src/resources/extensions/gsd/prompts/research-slice.md

@@ -42,7 +42,7 @@ An honest "this is straightforward, here's the pattern to follow" is more valuab
 
 Research what this slice needs. Narrate key findings and surprises as you go — what exists, what's missing, what constrains the approach.
 0. If `REQUIREMENTS.md` was preloaded above, identify which Active requirements this slice owns or supports. Research should target these requirements — surfacing risks, unknowns, and implementation constraints that could affect whether the slice actually delivers them.
-1. If a `GSD Skill Preferences` block is present in system context, use it to decide which skills to load and follow during research, without relaxing required verification or artifact rules
+1. **Load relevant skills.** Check the `GSD Skill Preferences` block in system context and the `<available_skills>` catalog in your system prompt. `read` any skill files relevant to this slice's technology stack before exploring code. Reference specific rules from loaded skills in your findings where they inform the implementation approach.
 2. **Skill Discovery ({{skillDiscoveryMode}}):**{{skillDiscoveryInstructions}}
 3. Explore relevant code for this slice's scope. For targeted exploration, use `rg`, `find`, and reads. For broad or unfamiliar subsystems, use `scout` to map the relevant area first.
 4. Use `resolve_library` / `get_library_docs` for unfamiliar libraries — skip this for libraries already used in the codebase