docs: README with practical examples, AGENTS.md context editing guide, config permissions

[e6qu]

Issue for this PR

N/A — documentation and configuration.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

README.md — rewritten with practical examples showing how to use each context editing feature:

• Sample conversations for hide, replace, externalize, annotate
• Edit history navigation (log, checkout, fork)
• Side thread parking and listing
• Focus agent behavior and triggers
• Tools reference table with safety constraints
• Full agent table (visible + hidden)

AGENTS.md — added context editing section:

• When and how to use context_edit (hide stale results, replace errors, externalize verbose output)
• When to use thread_park (defer off-topic findings instead of chasing them)
• When to use context_history (review and navigate edit DAG)

.opencode/opencode.jsonc — granted allow permissions for all context editing tools:

• context_edit, context_deref, context_history, thread_park, thread_list

How did you verify your code works?

Documentation-only change. No code modified.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR