Add official Discord bot research intake

[HarperZ9]

What changed

• Adds bounded Discord channel/thread intake through Discord's official bot REST API.
• Adds guild channel discovery plus active-thread capture without personal tokens, selfbots, or browser-session scraping.
• Keeps the bot credential environment-only and outside URLs, receipts, and captured item text.
• Registers the direct provenance method, wires run-config sources, documents usage, and adds focused adapter tests.
• Carries forward the approved collector design and implementation plan that were already committed locally but not published.

Why

This recovers the interrupted official-bot research-intake lane into an isolated, reviewable branch while preserving Gather's provenance and privacy boundaries.

Validation

• python -m pytest: 424 passed
• python -m ruff check src tests examples: passed
• python -m mypy src/gather: passed (56 files)
• python -m pytest -q --cov=src/gather --cov-report=term-missing --cov-fail-under=78: 424 passed, 85.24% coverage
• gather status --json: MATCH
• gather doctor --json: MATCH
• git diff --check: passed

The public-surface sweeper reports 37 em-dash findings in pre-existing design documents; the same 37 findings reproduce on the clean base checkout and were not introduced by the adapter commit.