Add dedicated abandoned outcome enum + consistent episode filtering

[tfatykhov]

From PR #47 Code Review (P2 findings)

P2-1: Fragile abandoned sentinel for decisions

Currently abandoned decisions use outcome="failure" + confidence=0.0 as a sentinel. This is fragile - a real failed decision with legitimately 0% confidence would be misclassified as abandoned.

Episodes already have a dedicated outcome="abandoned" value. Decisions should align:

• Add "abandoned" to the Decision outcome enum
• Update Brain.abandon() to set outcome="abandoned" directly
• Simplify the filter in _query() from NOT (outcome="failure" AND confidence=0.0) to outcome != "abandoned"

P2-2: Inconsistent episode filtering

_list_recent() accepts an outcome param that allows overriding the abandoned filter, but _search() and _search_recent_by_embedding() always hardcode outcome != "abandoned" with no override.

All three methods should have consistent filterability — either all allow override or none do.

Related

• PR fix: filter abandoned decisions/episodes from context recall (#45) #47 (merged) — context recall quality fixes
• Context recall quality: filter abandoned, deduplicate, limit verbosity #45 — original issue