Deprecate the repo due to pivot

[XinweiHe]

See traceroot-ai/traceroot#365 for details.

[greptile-apps]

Greptile Summary

This PR deprecates the traceroot-sdk-java repository by prepending a single-line plain-text deprecation notice to README.md, signalling that the project is no longer maintained following a product pivot (referenced in traceroot-ai/traceroot#365).

• Adds ⚠️ THIS PROJECT IS DEPRECATED AND NO LONGER MAINTAINED. as the very first line of README.md.
• No code, configuration, or dependency changes are included.
• The notice lacks a link to the upstream issue or any guidance on what users should migrate to, which may leave existing consumers without a clear next step.

Confidence Score: 5/5

• This PR is safe to merge — it is a documentation-only change with no runtime impact.
• The change is a single-line addition to a markdown file. There are no code, logic, or configuration modifications that could introduce regressions. The only concern is a missing migration/redirect link for users, which is a non-blocking style suggestion.
• No files require special attention.

Important Files Changed

Filename	Overview
README.md	Adds a plain-text deprecation banner as the first line of the README; no code or logic changes.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[User discovers traceroot-sdk-java] --> B{README banner visible?}
    B -- Yes --> C[⚠️ Deprecation notice shown]
    C --> D{Link to issue/alternative present?}
    D -- No --> E[User has no clear next step]
    D -- Yes --> F[User navigates to replacement/issue]
    B -- No --> G[User continues using deprecated SDK unknowingly]

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_{Last reviewed commit: 1265c1a}