[enhancement] Support MCP environment variable configuration for agent sessions

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Background

The Copilot SDK v0.1.25 (Feb 18, 2026) includes important MCP (Model Context Protocol) environment variable improvements in PR #484 authored by Steve Sanderson. The change fixes MCP environment variable handling by sending envValueMode: direct across all SDK implementations.

Key changes:

• Updated @github/copilot to 0.0.411-1 with MCP env var fixes
• CLI now supports direct environment variable mode for MCP servers
• Ensures environment variables are properly passed to MCP tools

Related SDK commits:

• Fix MCP env vars: send envValueMode direct across all SDKs
• Bump CLI dep to 0.0.411

Proposal

Add support for configuring MCP environment variables when creating Copilot agent sessions in Planeteer. This would allow users to:

1. Specify environment variables needed by MCP tools (e.g., API keys, database URLs, service endpoints) in the plan definition or a configuration file
2. Pass these environment variables to agent sessions via SessionConfig when executing tasks
3. Support both global environment variables (apply to all tasks) and task-specific environment variables

Implementation approach:

• Extend the Task model to include optional env field (key-value pairs)
• Update src/services/executor.ts to merge global and task-specific env vars when creating sessions
• Add UI in the refine screen for users to configure environment variables for tasks
• Store environment variables in the persisted plan JSON (with warnings about sensitive data)

Benefit

This enhancement enables Planeteer to work with Copilot agents that rely on MCP tools requiring configuration:

• MCP tool support: Many MCP servers need API keys, database credentials, or service endpoints passed as environment variables
• Task customization: Different tasks can use different configurations (e.g., dev vs. prod database URLs)
• Professional workflows: Supports real-world scenarios where agents need access to external services (databases, APIs, cloud providers)
• SDK alignment: Takes advantage of the recent SDK fixes ensuring environment variables are properly propagated

Acceptance Criteria

• Add env?: Record<string, string> field to the Task interface in src/models/plan.ts
• Update src/services/executor.ts to accept environment variables in session configuration when creating agent sessions
• Display environment variables in the refine screen task editor (with masking for sensitive values)
• Save environment variables to plan JSON with a warning comment about not committing secrets
• Add a settings file (.planeteer/settings.json) for global environment variables that apply to all plans
• Update SDK dependency to ^0.1.25 in package.json to ensure MCP env var fixes are available
• Document environment variable configuration in README with examples
• Add validation to warn users when environment variables look like secrets (contain "key", "token", "password", etc.)

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