Fix dispatch guard regressions and safe ticket parsing

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Summary

• Preserve in-memory budget/max-turn exhaustion guards across poll ticks so exhausted active tickets cannot redispatch in a loop when no tracker transition is configured or persistence fails.
• Carry retry-queued issue snapshots so touched-file conflict checks also block overlaps during continuation/retry delays.
• Make file-ticket front-matter recovery parse-only on read paths to avoid read-side atomic rewrites racing with agent/operator edits.

Type

• Bug fix
• Feature
• Docs
• Tests or tooling
• Refactor

Verification

Paste the commands you ran and the result.

/tmp/symphony-validate-venv/bin/python -m pytest tests/test_orchestrator_dispatch.py::test_max_total_turns_exhaustion_survives_next_tick_prune tests/test_orchestrator_dispatch.py::test_on_worker_exit_hit_max_turns_survives_next_tick_prune tests/test_orchestrator_dispatch.py::test_conflict_pre_check_blocks_retry_queued_overlap tests/test_tracker_file.py::test_parse_ticket_file_auto_heals_markdown_inside_front_matter -q
# 4 passed

/tmp/symphony-validate-venv/bin/python -m pytest tests/test_orchestrator_dispatch.py tests/test_tracker_file.py -q
# 120 passed

PATH="/tmp/symphony-test-bin:$PATH" /tmp/symphony-validate-venv/bin/python -m pytest -q
# 800 passed, 5 skipped, 2 warnings

For workflow/service/board changes:

PATH="/tmp/symphony-test-bin:$PATH" /tmp/symphony-validate-venv/bin/symphony doctor ./WORKFLOW.md
# FAIL: agent.kind=claude ('claude' not on PATH)
# FAIL: tracker.board_root (/workspace/kanban does not exist)
# other checks PASS

Risk Notes

• _turn_budget_exhausted remains process-local and requires an operator/service reset or configured terminal transition for recovery; this matches the existing safety guard and prevents unbounded redispatch loops.
• The front-matter recovery still lets reads parse healable tickets but no longer persists the healed shape unless a write path later updates the ticket.

Checklist

• This PR targets dev.
• I added or updated tests for behavior changes.
• I updated docs/examples for user-facing changes.
• I did not commit secrets, logs, local run state, virtualenvs, or generated artifacts.
• CI is green or I explained any expected failure above.