fix(sexpr): show field-equality example in filter parse error (REQ-142, #381)

[avrabe]

Closes the discoverability friction I filed in #381.

The s-expression filter dialect (shared by query, list --filter, export --filter, modify --where) puts an operator in head position, but the most common first attempt is (status "draft") — field name in head — which failed with only a list of head forms and no example of the correct shape.

Before:

note: unknown head symbol; see docs/getting-started.md for the supported forms (!=/…/or/…)

After:

note: unknown head symbol — the head is an operator, not a field name. For field
      equality use `(= <field> "<value>")`, e.g. `(= status "draft")` or
      `(and (= type "requirement") (has-tag "safety"))`. See docs/getting-started.md
      for all supported forms (!=/…)

The note is generated once in sexpr_eval, so the improvement reaches every command that parses a filter — verified on both modify --where and list --filter. Also added an example to the export --filter help (it had none, unlike list/query).

Verification

• Reproduced the exact s-expr filter discoverability: --where/--filter/query show no example of the common field-equality form #381 mistake ((status "draft")) and confirmed the note now shows (= status "draft").
• sexpr_eval::tests::parse_error_unknown_head_surfaces_note extended to assert the example is present; passes. clippy --all-targets + fmt clean; rivet validate PASS.

Implements: REQ-142

🤖 Generated with Claude Code

[github-actions]

📐 Rivet artifact delta

Change	Count
Added	1
Removed	0
Modified	0
Downstream impacted (depth ≤ 5)	0

Graph

graph LR
  REQ_142["REQ-142"]:::added
  classDef added fill:#d4edda,stroke:#28a745,color:#155724
  classDef removed fill:#f8d7da,stroke:#dc3545,color:#721c24
  classDef modified fill:#fff3cd,stroke:#ffc107,color:#856404
  classDef overflow fill:#e2e3e5,stroke:#6c757d,color:#495057,stroke-dasharray: 3 3

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Added

• REQ-142

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[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Rivet Criterion Benchmarks'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: f74197b	Previous: 1363ca7	Ratio
store_insert/10000	17961173 ns/iter (± 1906862)	13111470 ns/iter (± 466967)	1.37
link_graph_build/10000	42837872 ns/iter (± 6090357)	26585904 ns/iter (± 2402999)	1.61
validate/10000	23087438 ns/iter (± 2794722)	13014679 ns/iter (± 1149462)	1.77
diff/10000	10978597 ns/iter (± 1130411)	7521663 ns/iter (± 119826)	1.46

This comment was automatically generated by workflow using github-action-benchmark.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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