Add /speckit.converge SDD artifacts and project scaffolding

Dogfood the converge feature through Spec Kit's own workflow:

- spec.md, plan.md, tasks.md, research, data-model, contracts, quickstart
- requirements checklist for the feature
- ratified constitution v1.0.0 (.specify/memory)
- Specify project scaffolding (.specify/, .github agent + prompt files)

Defines a built-in /speckit.converge command that assesses spec/plan/tasks
against the codebase and appends remaining work as new tasks (no git, no
change tracking, append-only). Implementation not yet started.

Excludes unrelated working-tree changes to agents.py, extensions.py,
test_extensions.py, catalog.community.json, and README.md.

.github/prompts/speckit.agent-context.update.prompt.md

@@ -0,0 +1,3 @@
+---
+agent: speckit.agent-context.update
+---

.github/prompts/speckit.analyze.prompt.md

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+---
+agent: speckit.analyze
+---

.github/prompts/speckit.bug.assess.prompt.md

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+---
+agent: speckit.bug.assess
+---

.github/prompts/speckit.bug.fix.prompt.md

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+---
+agent: speckit.bug.fix
+---

.github/prompts/speckit.bug.test.prompt.md

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+---
+agent: speckit.bug.test
+---

.github/prompts/speckit.checklist.prompt.md

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+---
+agent: speckit.checklist
+---

.github/prompts/speckit.clarify.prompt.md

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+---
+agent: speckit.clarify
+---

.github/prompts/speckit.constitution.prompt.md

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+---
+agent: speckit.constitution
+---

.github/prompts/speckit.implement.prompt.md

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+---
+agent: speckit.implement
+---

.github/prompts/speckit.plan.prompt.md

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+---
+agent: speckit.plan
+---

.github/prompts/speckit.specify.prompt.md

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+---
+agent: speckit.specify
+---

.github/prompts/speckit.tasks.prompt.md

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+---
+agent: speckit.tasks
+---

.github/prompts/speckit.taskstoissues.prompt.md

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+---
+agent: speckit.taskstoissues
+---