feat(daemon): prune dead sessions past retention limits on startup

[mgabor3141]

Implements review ticket T22.

Dead-but-not-dismissed sessions previously persisted forever — each keeps a meta.json plus up to ~2 MiB of rotated scrollback under ~/.local/state/gmux/sessions/<id>/, removed only on explicit dismiss or orphan-sweep. For heavy users who never dismiss, the state dir grew unbounded. This adds a retention policy applied during the startup Sweep: dead-session corpses older than MaxAge are aged out, then only the newest MaxCount survive (LRU by effective timestamp — ExitedAt, falling back to LastActivityAt then CreatedAt). Defaults are 30 days / 200 sessions, overridable via GMUX_SESSION_RETENTION_DAYS and GMUX_SESSION_RETENTION_MAX (0 disables a limit).

Safety rules: Sweep only loads Alive=false records, and any still-running runner re-registers as alive immediately afterwards, so live sessions are never on the chopping block. Records with no parseable timestamp are treated conservatively — never aged out and ranked as newest for the count cap — so they are never evicted unexpectedly. The clock is injectable (WithClock) for deterministic tests; the policy is injected via WithRetention, and New without options preserves the prior no-prune behavior.

Verification

• cd services/gmuxd && go build ./... — passes
• go test ./... — all packages pass, including new sessionmeta retention tests (age-out, count cap, no-policy, undatable-survives, env override)

Source finding: L1

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Try this PR

curl -sSfL https://gmux.app/install-pr.sh | sh -s -- 296

Built from 7a18b00 — fix(daemon): reject overflowing GMUX_SESSION_RETENTION_DAYS
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[mgabor3141]

@greptile review

[greptile-apps]

Confidence Score: 5/5

Safe to merge — the retention logic is well-scoped to dead sessions only, the overflow guard is correct, and live sessions are protected by the existing Sweep contract.

The prune path is exercised by five focused tests with an injected clock, the env-override overflow guard is correct and regression-tested, and the wiring in main.go is a minimal one-liner. No changes touch live-session handling or existing persistence contracts.

No files require special attention.

Important Files Changed

Filename	Overview
services/gmuxd/internal/sessionmeta/sessionmeta.go	Core retention logic — adds RetentionPolicy, DefaultRetention (with env overrides + overflow guard), prune (age-out then count cap), and effectiveTime. Logic is sound; minor dead-code nit in prune's clock setup.
services/gmuxd/internal/sessionmeta/sessionmeta_test.go	Five new retention tests covering age-out, count cap, no-policy, undatable-survives, and env overrides. Missing a case for GMUX_SESSION_RETENTION_MAX=0 disabling the count limit, but all happy paths are well covered.
services/gmuxd/cmd/gmuxd/main.go	One-line wiring change: passes DefaultRetention() to New via WithRetention; clean and minimal.
packages/paths/paths.go	Doc comment only — adds retention policy description to SessionDir; accurate and matches the implementation.

Sequence Diagram

sequenceDiagram
    participant main as gmuxd/main
    participant store as Store.Sweep
    participant prune as Store.prune
    participant disk as Disk (sessions/)

    main->>store: Sweep()
    store->>disk: ReadDir(sessions/)
    disk-->>store: []DirEntry
    loop each entry
        store->>disk: Read meta.json
        disk-->>store: "Session (Alive=false)"
    end
    store->>prune: prune(loaded)
    alt "MaxAge > 0"
        prune->>prune: "cutoff = now() - MaxAge"
        loop each session
            alt "effectiveTime < cutoff"
                prune->>disk: Remove(sessDir) [age eviction]
            else
                prune->>prune: add to survivors
            end
        end
    end
    alt "MaxCount > 0 and len(survivors) > MaxCount"
        prune->>prune: sort survivors newest-first
        loop sess in survivors[MaxCount:]
            prune->>disk: Remove(sessDir) [count eviction]
        end
        prune->>prune: truncate to MaxCount
    end
    prune-->>store: survivors
    store-->>main: []Session (pruned)

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