Default capture_env to WB_* instead of capturing nothing

[duncankmckinnon]

Summary

• Capture only ran when capture_env patterns resolved to something, which required either THIRDEYE_CAPTURE_ENV exported into the hook process or an explicit capture_env in config.yaml. Neither is guaranteed — workbench dispatches agents from contexts that may not source an rc file, and a fresh install has no capture_env at all. The result was a silent no-op: no tags, no wb.* attributes, and nothing to say why.
• WB_* is thirdeye's own vocabulary (env_to_tag already strips the prefix when shaping tags), so it is the sensible built-in default rather than something a user has to discover and configure.
• A present capture_env key stays authoritative even when empty — that is the opt-out — and only an absent key takes the default. write_capture_env_patterns therefore persists [] rather than removing the key, since removing it would fall back to the default and leave capture-env clear unable to clear. capture-env show now reports built-in default as the source when no key is set.

Note: this is hardening, not the fix for the cursor-only symptom that prompted it — that root cause was workbench's inject_env defaulting (duncankmckinnon/workbench#69). This closes the adjacent failure mode where capture silently does nothing on a machine that never configured a pattern.

Test plan

• pytest tests/ — 2732 passed
• New/updated tests: absent key → ("WB_*",); capture_env: [] → (); THIRDEYE_CAPTURE_ENV still overrides; clear persists [] and keeps capture off; claude and codex session_start tag WB_* with no pattern configured, and write no tags under an explicit empty capture_env
• End-to-end: thirdeye-codex-session-start with THIRDEYE_CAPTURE_ENV unset and no capture_env in config now persists the captured env and writes plan-/task-/agent-/step- tags

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