fix(storage): exclude zero-valued Codex shell echoes from unfiltered search and relax IDF

[pablontiv]

Summary

Fixes the Codex shell wrapper half of the zero-valued search_echo query-time gap — the same bug class fixed for exec_command in #86 (bs-echo-exec-cmd-gap-r1).

#87 taught the requeue path (unmarkedDirectSearchCallSQL + pendingSearchEchoShellMatches + internal/directsearch) to recognize Codex shell command=["<sh>","-c"|"-lc","backscroll search ..."] rows stored as search_echo=0. But the functions that exclude such a row from unfiltered results right now, before an owner sync replays the source, were never touched:

• isDirectBackscrollSearchEcho (Go) — used by excludeDirectBackscrollSearchEchoes / refillCandidatesWithoutDirectEchoes for unfiltered search --text result pages
• directBackscrollSearchEchoSQL (SQL) — used by recallFrequency for --relax IDF document-frequency counting

Both recognized only the bash command=backscroll search and exec_command cmd=backscroll search three-token prefixes.

Reproduction (spike, hermetic)

New E2E TestZeroValuedCodexShellEchoExcludedBeforeReplay (following echo_zero_replay_e2e_test.go) ingests real Codex shell rollouts through the actual CodexReader (serialized shape asserted), forces search_echo=0 to reproduce the pre-#80 state, then searches as a startup-lock follower so no owner replay can converge the rows first. Before the fix:

• 8 shell echo rows leaked into the unfiltered search --text "violet handshake" page (ranks 2–9), while the identically-treated exec_command control rows were correctly excluded
• search --relax --text "violet handshake adaptation" returned zero rows: the leaked shell rows inflated DF(violet)/DF(handshake) to 9 > DF(adaptation)=4, inverting the drop order

Fix

Reuses the already-reviewed PR #87 logic instead of a new SQL/GLOB implementation:

• Go: isDirectBackscrollSearchEcho gains a shell branch calling the shared pendingSearchEchoShellMatches (locates the command= token inside the sorted key=value list, json.Decoder-decodes exactly one JSON array value, feeds directsearch.IsCodexDirectSearchCall — the exact predicate the reader uses at ingest).
• SQL/IDF: directBackscrollSearchEchoSQL deliberately stays pure-prefix (the JSON argv's separator byte-sequences are provably unbounded for SQL GLOB — the lesson of PR fix(search): requeue search_echo=0 Codex shell wrapper calls #87's three failed enumeration rounds). recallFrequency now subtracts shell echo rows using the same broad-SQL-prefilter (text LIKE 'shell %') + strict-Go-predicate split, so unfiltered IDF counts exactly the row set the Go predicate keeps.
• Docs: docs/search.md#query-echo-handling and the --relax IDF rule now enumerate the shell argv form and document the query-time zero-valued fallback.

Regression coverage

• E2E (real reader round-trip, forced-zero state, follower lock): unfiltered page exclusion + --relax IDF drop order, with exec_command control in the same state
• TestRecallFrequencySQLEchoPredicatePreservesBoundaries: 11 new shell rows asserting Go predicate, SQL predicate, and recallFrequency DF per row — including false-positive guards (wrong command, wrong flag -x, 4-element argv, backscroll status, env-wrapper and absolute-path inside argv, NBSP separator via real json.Marshal)
• TestRelaxationUnfilteredIDFExcludesZeroValuedCodexShellEchoes: dedicated IDF + relaxed-recall test
• TestRecallFrequencyUnfilteredSQLMatchesGoScan: shell echo rows added to the 8000-row SQL-vs-Go-scan agreement check
• Mutation-checked: with the fix stashed, all new tests fail RED; existing Claude/exec_command/NULL/search_echo=1 cases stay green in both states

just ci green (aggregate coverage 86.6% ≥ 85%).

Note: TestSearchRelaxationQueryEchoesDoNotInvertIDF (pre-existing, untouched here) has a latent fragility — its strings.Contains(out, "echo-") guard trips when GOTMPDIR contains the substring echo- (e.g. a worktree/branch named fm/bs-echo-...). Environmental, not caused by this change; flagged for a possible follow-up.