chore: remove legacy agent template ID compatibility after one release cycle

[jolestar]

Context

PR #2932 normalizes the general-purpose agent template IDs:

• holon-developer → software-developer
• holon-reviewer → code-reviewer
• holon-release → release-manager
• holon-github-solve → github-solver

The runtime currently preserves these old IDs with a narrow compatibility fallback so existing agent configurations continue to resolve during migration.

Removal policy

Keep the compatibility fallback for one full release cycle after the first release that contains #2932. Remove it no earlier than the following release cycle, after users have had a complete release window to migrate to the new IDs.

Scope

• Remove the legacy template ID mapping and fallback resolution from src/agent_template.rs.
• Remove or update migration-only tests and documentation.
• Confirm checked-in templates, examples, Actions, and CLI defaults no longer reference the four legacy IDs except in historical release notes.
• Add a release-note entry announcing removal and the required replacement IDs.

Acceptance criteria

• Identify and record the first release containing feat: normalize agent template names #2932.
• Confirm one full subsequent release cycle has elapsed.
• Repository search finds no active use of the four legacy IDs.
• Legacy fallback code and migration-only tests are removed.
• Template tests, formatting, and warning-free all-target build pass.