docs: add trace model recommendation plan

[kmishra1204]

Summary

Adds a planning document for issue #12 so the working group can prepare the Agent Behavior Trace Model recommendation without pre-committing to a new specification.

The plan is intentionally decision-neutral. It starts from candidate existing standards and models, defines comparison criteria, names the inputs needed before a recommendation, and documents an adopt / extend / new-artifact decision path.

Issue #12 acceptance criteria

• Candidate models and standards are listed for comparison.
• Evaluation criteria are documented.
• Required inputs from prior work, use cases, taxonomy, gap analysis, and focus groups are identified.
• The decision path for adopt / extend / new specification is documented.
• The review process and target milestone are documented.

Validation

• git diff --check
• ASCII check for TRACE_MODEL_RECOMMENDATION_PLAN.md

Closes #12.

[kmishra1204]

Validated this current head after fetching upstream main.

Local checks:

• Branch is two commits ahead and zero behind upstream main
• git diff --check origin/main...fork/codex/trace-model-plan-issue-12
• Changed-file scope is markdown-only: TRACE_MODEL_RECOMMENDATION_PLAN.md, README.md
• Relative links in changed markdown resolve inside the branch
• No TODO/FIXME/example placeholder text found in changed files