feat(github): add configurable floor for progress-comment edit cadence

[aparajon]

Why this matters

The built-in edit cadence is a fixed policy. During a GitHub rate-limit incident — or on a server hosting many concurrent long-running applies — operators need one valve that slows every apply's progress-comment traffic at once, without a code change or redeploy of new logic.

What it does

Adds a server config setting that floors the built-in age-decayed cadence:

progress_comment_min_edit_interval: 2m  # Go duration; default unset (no floor)

• No progress comment is edited more often than the configured interval, regardless of the built-in cadence.
• State changes still publish immediately — the floor spaces out progress noise, not state transitions.
• The cadence the progress comment advertises in its "Last updated" footer reflects the floor, so PR readers see the configured interval rather than wondering why updates slowed down.
• Invalid or negative values are rejected at config validation; an invalid value read at runtime logs a warning and falls back to the built-in cadence.
• Documented in docs/configuration.md; users needing high-resolution progress use the CLI, which polls the server directly and is unaffected.

Stacked on #758.

🤖 Generated with Claude Code