fix(doctor): use live open-fd counts for accurate fd exhaustion eta #115

[Rakshak05]

Closes #115

What

This PR fixes file descriptor exhaustion ETA estimations by collecting and utilizing the actual live open file descriptor count and soft limits from /proc/<pid>/fd and /proc/<pid>/limits for top leaking processes rather than system-wide window net deltas.

How

• Collector Updates: Extended FDEntry and FDSnapshot models to include live FD counts and limit fields, and populated them in Snapshot() for the top 5 net-delta processes.
• Proc Parsers: Implemented safe, side-effect-free /proc parsing helpers (CountProcFDs and ReadProcFDLimit) that utilize a mockable package-level base path to ensure tests can run reliably on non-Linux platforms.
• Headroom Calculations: Created a shared fdHeadroom function to calculate exact remaining headroom with prioritized fallback strategies when /proc metrics are unavailable.
• Refactoring & UI: Updated evalFDLeak and predictFDExhaustion to use fdHeadroom, appending a clear "(estimated from window delta — actual may be lower)" warning to the impact string when the live count cannot be measured.

Testing

• go build ./... passes
• go test ./... passes
• go vet ./... passes
• golangci-lint run ./... passes
• Tested locally with: Unit tests mocking /proc structures and rules/predict engines.

• N/A — pure docs/refactor
• sudo ./bin/bpf-verify --read 5s confirms 6/6 programs still load
• ./scripts/verify.sh passes (or specific phase: ./scripts/verify.sh quality)

Checklist

• PR title follows Conventional Commits (feat(scope): subject)
• All commits are DCO-signed (git commit -s)
• No unrelated changes pulled in
• Documentation updated where user-visible behavior changed
• Added/updated tests for new code paths
• If a new doctor rule, paired with a chaos scenario in scripts/verify.sh

[github-actions]

🚀 First PR — welcome aboard!

A few things to expect:

1. CI: every PR runs build + race tests + lint + (eventually) the kernel matrix. If something fails, the log will tell you exactly which gate.
2. DCO: every commit needs Signed-off-by: — git commit -s adds it automatically.
3. Conventional Commits: PR titles like feat(doctor): add new rule or fix(bpf): handle X. We squash-merge by default.
4. Review: a maintainer will review within 72 hours. Suggestions are conversations, not orders — push back if something doesn't fit your context.

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