[aw][code health] Code clarity nits: dead EventBase model, confusing duration function names, non-idiomatic default_factory

[microsasa]

Three small, related clarity issues — bundled because none alone justifies a separate issue.

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1. EventBase is dead code (models.py:215–222)

EventBase is defined with the comment "useful for filtering / indexing" but is never imported or used in any production code (src/). It partially duplicates SessionEvent's envelope fields. It adds maintenance burden with no payoff.

Fix: Remove EventBase from models.py and its single test reference in test_models.py.

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2. Confusing duration-formatter names (report.py:61 and report.py:91)

Two functions have nearly identical names but do fundamentally different things:

Name	Signature	What it does
format_duration (public)	(ms: int) → str	Formats a millisecond count as "6m 29s"
_format_duration (private)	(start: datetime) → str	Computes elapsed time from start to now, formats as "Xh Ym"

The private one should be renamed to _format_elapsed_since (or similar) to make the distinction obvious at call sites.

Fix: Rename _format_duration → _format_elapsed_since throughout report.py.

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3. Non-idiomatic default_factory for toolRequests (models.py:107)

# current — confusing: list[dict[str, object]] is a type, calling it returns []
toolRequests: list[dict[str, object]] = Field(
    default_factory=lambda: list[dict[str, object]]()
)

# idiomatic
toolRequests: list[dict[str, object]] = Field(default_factory=list)

The current form works because list[X]() delegates to list(), but it reads as if the type annotation is being instantiated, confusing contributors.

Fix: Replace with default_factory=list.

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Testing Requirements

• After removing EventBase: ensure all tests still pass; remove the EventBase test case from test_models.py.
• After renaming _format_duration: update any test references to the private name and ensure render_live_sessions / render_full_summary still render Running column correctly (covered by existing tests).
• After fixing default_factory: no behaviour change; existing model-parsing tests should confirm toolRequests still defaults to [].

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