Add --select mode to implementation workflow

[martymcenroe]

Problem

Currently run_implement_from_lld.py requires --issue ISSUE to be specified explicitly. Users must know which issues have approved LLDs.

Proposed Solution

Add --select flag that:

1. Scans docs/lld/active/ for approved LLDs
2. Displays a numbered list with issue title and approval date
3. Prompts user to select one
4. Runs the workflow on the selected issue

Example

$ python tools/run_implement_from_lld.py --select

Approved LLDs ready for implementation:
  1. #147 - Add retry logic to API calls (approved 2026-01-28)
  2. #166 - Implement caching layer (approved 2026-01-30)
  3. #180 - Add webhook support (approved 2026-02-01)
  ...

Select issue [1-7]: 2

Running TDD workflow for #166...

Acceptance Criteria

• --select flag shows list of approved LLDs
• List shows issue number, title, and approval date
• User can select by number
• Workflow proceeds with selected issue
• --select is mutually exclusive with --issue