Clarify and de-duplicate Copilot instructions across the repo

[Copilot]

This cleans up Copilot guidance so it matches the current repository workflows and tool surface. The main changes remove inaccurate commands, eliminate stale tool references, and consolidate repeated guidance where the same rule was expressed multiple ways.

• Shared monorepo guidance

  • Replaced invalid or misleading command examples in .github/copilot-instructions.md
  • Standardized changelog guidance on pnpm chronus add
  • Clarified watch-mode guidance so it points to package-local test/watch commands instead of a nonexistent root script

• Emitter-specific instruction cleanup

  • Consolidated repeated changelog instructions in http-client-java.instructions.md
  • Simplified Java release branch guidance to align with the repo-level publish/ convention
  • Kept emitter-specific setup intact while removing redundant wording

• Prompt accuracy fixes

  • Updated testserver-generation.md to use the explicit Chronus command for @typespec/http-specs
  • Removed references to unavailable agent/tool APIs from upgrade-tcgc.instructions.md
  • Reframed the TCGC upgrade flow around a plain checklist instead of nonexistent task-management tools

• Noise reduction

  • Removed out-of-band behavioral instructions from packages/http-client-js/.github/copilot-instructions.md that were not about repository development
  • Tightened wording where the same intent was repeated across files

Example of the command normalization applied in the docs:

- Add change description: `pnpm change add`
+ Add a change description with `pnpm chronus add`

[azure-pipelines]

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