refactor(adapters): chi-style middleware via WithAdapters

[klaidliadon]

Reshapes the v0.2 adapters into chi-style middleware. Each adapter is now a config-only constructor that returns a runnable.Adapter, and runnable.WithAdapters wraps the runnable's runFunc with the supplied list (first-listed = outermost). Work stays positional on runnable.New; adapters become decoration that composes with other Options.

Closes the marino39 review item asking for a chi-style middleware factoring. Stacked on top of #4 — review against feat/drain-and-ticker, not master.

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Commits

• f7fa106 — RunFunc/Adapter types and WithAdapters Option in core; Draining and Ticker reshaped as runnable.Adapter constructors.
• 2432924 — Recovering and Retry migrated to adapters; WithRecoverer and WithRetry removed; Status.Restarts dropped (it counted retry re-entries via an onStart side-channel that doesn't survive the migration).
• 768f979 — README and adapters doc comment rewritten for the WithAdapters shape.

Shape

r := runnable.New(reconcile, runnable.WithAdapters(
    adapters.Draining(10*time.Second),
    adapters.Recovering(panicHandler),
    adapters.Retry(3, time.Minute),
    adapters.Ticker(2*time.Second),
))

vs. v0.1:

r := runnable.NewTicker(2*time.Second, reconcile,
    runnable.WithDrain(10*time.Second),
    runnable.WithRecoverer(reporter, nil),
    runnable.WithRetry(3, time.Minute),
)

Test plan

• go test -race -count=1 ./... — passes (runnable 6.1s, runnable/adapters 2.8s).
• golangci-lint run ./... — clean.
• Each commit builds and tests green standalone (verified by stashing later commits and running build+test before each commit).

Notes

• runnable.Adapter lives in the core package so runnable.WithAdapters doesn't have to import the adapters subpackage. The adapters package depends on runnable, not the other way around.
• Status.Restarts is gone, pending a proper event/observer hook in a later release. Documented in the migration section of the README.

[klaidliadon]

Superseded — content cherry-picked into #4 (commits b80a033, 92b5063, c5f4101). PR #4 now ships the final v0.2 chi-style middleware shape end-to-end.