Add template support for issue creation (bd-164b)

- Built-in templates: epic, bug, feature (embedded in binary)
- Custom templates in .beads/templates/ (override built-ins)
- Commands: bd template list/show/create
- Flag: bd create --from-template <name> "Title"
- Template fields: description, type, priority, labels, design, acceptance
- Security: sanitize template names to prevent path traversal
- Flag precedence: explicit flags override template defaults
- Tests: template loading, security, flag precedence
- Docs: commands/template.md and README.md updated

Closes bd-164b

Amp-Thread-ID: https://ampcode.com/threads/T-118fe54f-b112-4f99-a3d9-b7df53fb7284
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/templates/epic.yaml

@@ -0,0 +1,51 @@
+# Built-in template for creating epics
+name: epic
+description: |
+  ## Overview
+  
+  [Describe the high-level goal and scope of this epic]
+  
+  ## Success Criteria
+  
+  - [ ] Criteria 1
+  - [ ] Criteria 2
+  - [ ] Criteria 3
+  
+  ## Background
+  
+  [Provide context and motivation]
+  
+  ## Scope
+  
+  **In Scope:**
+  - Item 1
+  - Item 2
+  
+  **Out of Scope:**
+  - Item 1
+  - Item 2
+
+type: epic
+priority: 1
+labels:
+  - epic
+
+design: |
+  ## Architecture
+  
+  [Describe the overall architecture and approach]
+  
+  ## Components
+  
+  - Component 1: [description]
+  - Component 2: [description]
+  
+  ## Dependencies
+  
+  [List external dependencies or constraints]
+
+acceptance_criteria: |
+  - [ ] All child issues are completed
+  - [ ] Integration tests pass
+  - [ ] Documentation is updated
+  - [ ] Code review completed