feat: argus gcscore — GC Health Score Card with A–F grade

[rlaope]

Closes #150.

Summary

argus gcscore <gc-log-file> — a one-page GC Health Score Card. Scores six KPI axes against widely-accepted thresholds and emits a weighted A–F grade plus up to three improvement hints.

Sample output:

┌─ GC Health Score ───────────────────────────────────────────┐
│  Overall: B  (good, minor tuning opportunities)  [82/100]   │
│                                                             │
│  Pause p99           ✓ Pass    82 ms   (target < 200 ms)    │
│  Pause tail (max)    ⚠ Warn   720 ms   (target < 500 ms)    │
│  Throughput          ✓ Pass   98.4%    (target > 95%)       │
│  Full GC frequency   ✓ Pass   0.2 /hour (target 0 / hour)   │
│  Allocation rate     ✓ Pass   280 MB/s (target < 1 GB/s)    │
│  Promotion ratio     ⚠ Warn   22.0%    (target < 20%)       │
│                                                             │
│  Hints:                                                     │
│   • Long tail pauses suggest concurrent-mode failure ...    │
│   • Survivor overflow promoting short-lived objects ...     │
└─────────────────────────────────────────────────────────────┘

Scope

• Log-based: argus gcscore <gc-log-file> (reuses GcLogParser / GcLogAnalyzer)
• --format=json for automation
• Axes: Pause p99, Pause tail (max), Throughput, Full GC frequency, Allocation rate, Promotion ratio
• Pass/Warn/Fail per axis, weighted overall → grade A (≥ 90) / B (≥ 75) / C (≥ 60) / D (≥ 40) / F (< 40)
• Up to 3 hints from a rule base keyed on which axes failed
• Fallback hint when no axis fails
• Missing rate-analysis data marks axes as N/A (excluded from weighted average)
• i18n (en, ko, ja, zh), shell completions (bash, zsh, fish, ps1)

Non-goals in this PR

• Live <host:port> target mode — deferred. Server-side metric shape for p99/p999 from live scrape is not yet exposed in a form directly usable here; will be tracked in a follow-up issue.
• Not auto-tuning — hints are suggestions only.

References (design lineage)

• GCeasy.io Report Score Card (1-page KPI summary)
• Censum (jClarity / Microsoft) rule-based recommendations with reasoning

Test plan

• ./gradlew :argus-cli:test --tests io.argus.cli.gcscore.* — passes
• ./gradlew :argus-cli:test full suite — passes
• Unit tests cover: perfect GC (A grade), long p99 downgrade, frequent Full GC verdict, N/A axes when rate data missing, high promotion ratio surfacing survivor hint, grade boundaries
• Manual smoke against a real GC log — pending a fixture log

Follow-up

• Live target mode (requires server-side p99 histogram exposure) — new issue to be raised.