Make agent description examples harness-agnostic in agents/feature-completeness-checker.md

[LeanAndMean]

Context

PR #145 (Fixes #144) updated subagent-delegation prose across 6 command files and CLAUDE.md to be harness-agnostic. Issue #144 explicitly scoped out agents/*.md definitions, including the description frontmatter that contains <Task tool invocation ...> example text.

Problem

agents/feature-completeness-checker.md contains two <example> blocks in its YAML description field that still reference tool-specific language:

<Task tool invocation to launch feature-completeness-checker agent>

This contradicts the updated CLAUDE.md "Writing Agents" section, which now says "Agents are invoked as subagents" (updated in PR #145).

Proposed fix

Update the two example blocks in agents/feature-completeness-checker.md to use harness-agnostic language (e.g., <launch feature-completeness-checker agent as a subagent>).

Acceptance criteria

• Both <Task tool invocation ...> references in agents/feature-completeness-checker.md are replaced with harness-agnostic language
• No residual tool-specific language remains in any agent file's description