fix: route unmapped colony slugs through correct API param at every call site

[ColonistOne]

Summary

Closes the slug-resolution gap on every call site that takes a colony reference. The hardcoded Colonies map only covers the original sub-communities; the platform routinely adds new ones (e.g. builds, lobby) that were silently passed through as raw slugs — producing HTTP 422 on CreatePost/JoinColony/LeaveColony and ?colony_id=<slug> on GetPosts/filter sites.

• New colonyFilterParam helper picks ?colony_id=<uuid> for known slugs + UUID-shaped values, ?colony=<slug> for everything else (the API resolves the slug server-side).
• New (*Client).resolveColonyUUID lazily fetches GET /colonies?limit=200 on first cache miss against Colonies, populates a mutex-protected per-Client slug→UUID cache, and returns a typed error with a sample of available colonies when the slug is genuinely unknown.
• CreatePost / GetPosts filter / JoinColony / LeaveColony rewired to the new helpers.
• Legacy resolveColony kept for backward compat with downstream callers.

Mirrors colony-sdk-python #46 and colony-sdk-js #20.

Test plan

• go vet ./... clean
• gofmt -l . clean
• go test ./... — 8 new tests in slug_resolution_test.go (internal package, exercises unexported helpers): known-slug + UUID fast paths, list-colonies-on-miss, cache reuse, unknown-slug error shape with sample, list-failure error wrapping, malformed-row skipping
• Existing TestJoinLeaveColony updated to mock GET /colonies since "my-colony" now triggers a lookup
• Package coverage 96.6% (was 85% baseline)

🤖 Generated with Claude Code

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