Let tools and features reuse definitions instead of duplicating them #151

Feature Description:

A tool and a feature cannot declare that they use the same software, credential
or network setup. When more than one extension needs the same agent, SDK or API
provider, each one carries its own install recipe and its own credential and
network declarations, and nothing relates the copies to each other.

Current cases:

• Provider declarations. The anthropic-api-key credential and its
  api.anthropic.com mapping are declared separately in five tool specs on
  main: claude, theia, theia-next, opencode and pi. OpenAI is declared
  in five specs and Gemini in four.
• Copilot. feat: Add GitHub Copilot CLI as a supported tool profile #90 #128 adds a copilot tool and feat: support GitHub Copilot and Claude Code in Theia profiles #127 adds a copilot-cli feature
  for Theia's Copilot provider. Both install the same npm package with separate
  recipes (@github/copilot and @github/copilot@latest). Only the tool
  declares a credential and network access.
• Claude. The claude tool installs the Claude Code CLI. Theia's Claude
  provider needs the Claude Agent SDK instead, which feat: support GitHub Copilot and Claude Code in Theia profiles #127 adds as a separate
  claude-agent-sdk feature. It has no relation to the claude tool and relies
  on the Anthropic declarations in the Theia specs.

What this causes:

• Copies drift. A version pin, allowlist entry or auth fix made in one copy does
  not reach the others. The two Copilot recipes already differ, and neither has
  merged yet.
• A tool and a feature that declare the same credential or environment variable
  cannot run together. error: resolve active secrets #109 hit this with the copilot tool and the
  github-cli feature, which both claimed GH_TOKEN.
• Every new agent or IDE integration adds another copy.