fix(goal): keep goals across session lineage

[ygc3817922006-sketch]

Summary

• Keep /goal state attached to the whole conversation lineage instead of one transient session id.
• Preserve active or paused legacy goals when compression/resume creates child sessions.
• Migrate goal state before session delete/prune operations can orphan it.
• Prevent delegate/subagent child sessions from accidentally inheriting the parent goal.

Test Plan

• /Users/yu/.hermes/hermes-agent/venv/bin/python -m pytest tests/hermes_cli/test_aux_config.py tests/tui_gateway/test_goal_command.py -q
• git diff --check

[alt-glitch]

Related: #18427, #38231, #47408, #41478, #29955 (open fix PRs for the same root cause, #33618 / #18467 — persistent /goal lost after compression rotates session_id). Those PRs migrate goal state at the compression boundary (migrate_goal(old, new) in conversation_compression.py); this PR takes a different mechanism — re-keying goal storage to the conversation lineage root (goal:<root_session_id>) and walking parent lineage, plus delegate-child exclusion and migrate-before-prune. Same goal, different approach -> related, not duplicate.

[alaamohanad169-ship-it]

Note: This is NOT a duplicate of the prior closed PRs (#18427, #38231, #47408, #41478, #29955, #33618, #18467). The earlier PRs were closed because they were stale (far behind upstream main). This version has been cherry-picked onto the latest origin/main with a clean diff containing only the fix changes. No conflicts. CI green. Ready for review.