[FEATURE] Support block/proc for dynamic context setup in Agent

[mikemikimike]

Closes #900

Summary:

• Allow Agent#context to accept a static Context or a deferred block/Proc.
• Resolve deferred contexts from declared inputs before building plain Agent chats and instances.
• Reuse the resolved Context during configuration so each build evaluates the block once.
• Preserve dynamic evaluation for existing Rails-backed chat records.

Validation:

• bundle exec rspec spec/ruby_llm/agent_dsl_spec.rb --tag ~live (27 examples, 0 failures)
• bundle exec rspec --tag ~live (2,543 examples, 17 existing Windows/environment failures; no changed-path failures)
• bundle exec rubocop lib/ruby_llm/agent.rb spec/ruby_llm/agent_dsl_spec.rb
• bundle exec archspec check
• git diff --check

[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.43%. Comparing base (959f42c) to head (6748c56).
⚠️ Report is 52 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/ruby_llm/agent.rb	95.83%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #903      +/-   ##
==========================================
- Coverage   94.44%   94.43%   -0.02%     
==========================================
  Files         238      238              
  Lines       11940    12085     +145     
  Branches     2030     2069      +39     
==========================================
+ Hits        11277    11412     +135     
  Misses        285      285              
- Partials      378      388      +10     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Copilot]

🟢 Approval recommended

The feature is implemented coherently with added specs, and the remaining feedback is limited to small documentation/efficiency fixes.

Pull request overview

This PR extends RubyLLM::Agent’s DSL so context can be configured with a deferred block/Proc, enabling per-run context setup based on declared inputs while ensuring plain (non-Rails) agent chats/instances resolve the context once before chat construction.

Changes:

• Allow Agent.context to accept either a static Context or a deferred Proc/block.
• Resolve deferred contexts from runtime inputs before building plain agent chats and instances, and reuse the resolved context during configuration to avoid double evaluation.
• Add specs covering deferred context resolution from inputs and “evaluate once” behavior.

File summaries

File	Description
lib/ruby_llm/agent.rb	Adds deferred context support, context pre-resolution for plain builds, and reuses resolved contexts during configuration.
spec/ruby_llm/agent_dsl_spec.rb	Adds test coverage for input-driven context blocks and single-evaluation behavior.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 2
• Review effort level: Lite

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.