fix: resolve upstream-sync.yml workflow failures

[Copilot]

Two bugs caused the upstream-sync.yml workflow to fail at the Setup Environment step with Invalid value. Matching delimiter not found 'UPSTREAM_PROMPT_DELIMITER', and a silent invalid-JSON bug in Create MCP Config.

Changes

• _upstream_agent_prompt.md — add trailing newline (POSIX convention). Without it, cat output the file flush against the heredoc delimiter, so GitHub Actions couldn't locate UPSTREAM_PROMPT_DELIMITER on its own line when parsing $GITHUB_ENV.

• Setup Environment step — add printf '\n' before the delimiter echo as a defensive guard against any future file lacking a trailing newline:

      cat "$GITHUB_WORKSPACE/.github/_upstream_agent_prompt.md"
  +   printf '\n'
      echo 'UPSTREAM_PROMPT_DELIMITER'

• Create MCP Config step — fix heredoc EOF indentation. The EOF marker was indented 2 spaces relative to cat in YAML, so after YAML block-scalar stripping it became EOF in the shell script — not at column 0, so bash read until end-of-file and appended EOF to the JSON, producing invalid output. Fix aligns EOF to the same YAML indentation level as cat so it lands at column 0 post-stripping.

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