fix(core): distinguish never-named sessions from named-then-cleared

[flora131]

Upstream 7bdb16c2 gave pi's storage a has_session_name / session_name
pair so a cleared title stops reading as "never named" (spec §5.6
gap 7). Atomic's JSONL already records the distinction durably — a
clear appends a session_info entry with name "", a never-named session
has none — but both readers collapsed it: getLatestSessionName reduced
everything to string | undefined with || undefined, and the
SessionInfo list reducer did the same, so no read surface could tell
the two states apart.

Read-side only; no storage change, no migration, and the SQLite
session backend is not adopted:

• SessionNameState ({ hasName, name? }) mirrors pi's pair and is
  exported from the package index alongside SessionInfo.
• getLatestSessionName returns the state; SessionManager.getSessionName
  keeps its bare-string contract via .name, and the new
  getSessionNameState() exposes the state (also picked into
  ReadonlySessionManager, with the matching create-harness stub).
• The SessionInfo list reducer records hasName alongside name.
• The picker's named filter still filters on the current name; its
  hasSessionName helper now documents that a cleared name filters as
  unnamed.

Named regression cleared-session-name-differs-from-never-named runs
against hand-written existing-shape session files and asserts both
readers (list reducer and SessionManager.open) keep the states apart,
that the bare-string surface is unchanged, and that the live name/clear
path still writes the clear as name "". docs/session-format.md
documents the durable distinction and the new reader.

Assistant-model: Claude Opus 5

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[flora131]

A tmux capture is not applicable to L17 (never-named versus named-then-cleared): this layer's behavior has no directly drivable interactive surface. Its evidence is the named regression tests in this PR plus the local gates — npm run check, the three vitest projects, and the coding-agent suite — all green on this branch.