fix(specs): raise comment body limit to 20k

What

Raise the spec comment body length limit from 4,000 to 20,000 characters in Hive.Specs.Comment.

Why

The 4,000-char cap was an arbitrary application-level guardrail with no storage backing — the spec_comments.body column is :text, so Postgres imposes no limit. The cap was:

• Cramped for non-human authors. Comments aren’t only typed in the dashboard form; they’re also posted by MCP clients/agents via the add_spec_comment tool. 4,000 chars (~600–800 words) is tight for an agent posting a detailed review or analysis.
• Inconsistent. The spec body itself allows 20,000, yet a comment discussing it was capped at a fifth of that.

What changed

• lib/hive/specs/comment.ex: validate_length(:body, min: 2, max: 4_000) → max: 20_000.

Notes for reviewers

• One-line change. The min: 2 floor is kept, so empty/one-char comments are still rejected.
• No migration needed — the column is already :text.
• No tests asserted the old 4,000 limit. mix compile --warnings-as-errors passes.
• Backend-only; nothing user-facing to document in the README.

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