fix(hermes-agent): create entry span from session source

[sipercai]

Summary

• Keep the latest Hermes official CLI, IM/gateway, TUI, and API Server paths covered by explicit AIAgent.platform ENTRY spans.
• Mirror Hermes session source resolution for platform-less AIAgent calls: platform, then HERMES_SESSION_SOURCE, then cli.
• Change no-platform top-level Hermes runs from AGENT-only to ENTRY -> AGENT, with tests and docs for the behavior.

Verification

• PYTHONPATH="instrumentation-loongsuite/loongsuite-instrumentation-hermes-agent/src:util/opentelemetry-util-genai/src:opentelemetry-api/src:opentelemetry-sdk/src:opentelemetry-semantic-conventions/src:opentelemetry-instrumentation/src" python -m pytest instrumentation-loongsuite/loongsuite-instrumentation-hermes-agent/tests/test_telemetry_spec.py -q (34 passed)
• git diff --check --cached
• python /Users/sipercai/project/ai_loop/team-skills/loongsuite-github-pipeline/scripts/check_loongsuite_pr_readiness.py --repo .
• tox -e precommit
• Latest Hermes acd7932 offline smoke:
  • no-platform AIAgent.platform=None produced ENTRY -> AGENT
  • API Server _create_agent() produced platform=api_server and ENTRY -> AGENT

Notes

EntryInvocation currently has no session-source field, so this PR only aligns ENTRY creation with Hermes's session source decision. It does not add a new custom span attribute for source labeling.

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