feat: add moss bench command for latency benchmarking (closes #433)

[JITHENTHIRIYA]

Pull Request Checklist

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• I have read the CONTRIBUTING guide.
• I have updated the documentation (if applicable).
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Description

Adds moss bench, a new top-level CLI command that runs a query workload against an index and reports p50/p95/p99 latency percentiles. Users can supply queries inline
(--query, repeatable) or from a file (--queries-file, one query per line), control the number of timed runs (--runs, default 20) and warmup iterations (--warmup,
default 3), and choose between local and cloud query modes. Output is a Rich table per-query plus an overall aggregate in human mode, or structured JSON with --json for
scripting and CI.

Fixes #433

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Changes

• packages/moss-cli/src/moss_cli/commands/bench.py — new command implementation with _percentile (linear interpolation, no extra dependencies) and bench_command
• packages/moss-cli/src/moss_cli/main.py — imports and registers bench_command as moss bench
• packages/moss-cli/tests/test_bench.py — 15 tests covering _percentile edge cases, CLI validation errors, and happy-path scenarios (no credentials required)
• packages/moss-cli/README.md — adds ### Benchmark section under Commands and a feature bullet in the Features list

Example Output

Loading index my-index locally...
Running 6 warmup + 40 timed iterations across 2 queries...

    Benchmark — my-index

┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━━━━┓
┃ Query ┃ p50 (ms) ┃ p95 (ms) ┃ p99 (ms) ┃ min (ms) ┃ max (ms) ┃
┡━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━╇━━━━━━━━━━╇━━━━━━━━━━╇━━━━━━━━━━╇━━━━━━━━━━┩
│ what is semantic search? │ 4.12 │ 5.88 │ 6.74 │ 3.91 │ 7.02 │
│ how does embedding work? │ 3.98 │ 5.21 │ 6.10 │ 3.75 │ 6.55 │
└──────────────────────────────┴──────────┴──────────┴──────────┴──────────┴──────────┘

Overall (40 runs)
p50: 4.05 ms
p95: 5.60 ms
p99: 6.48 ms
min: 3.75 ms max: 7.02 ms

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