feat: warn when system keyring is unavailable

[acaloiaro]

Summary

• Emit a one-time warning when age.usekeychain is enabled but no OS keyring provider is available, so users understand why passphrase caching won't persist across sessions
• Warn when writing a passphrase to the OS keyring fails at runtime
• The cacher.Set and askPass.Passphrase signatures now accept a context.Context to support the warning output
• Add tests for askPass initialization, osKeyring get/set paths, and the user-facing warning

[dominikschulz]

LGTM. Please fix the lint issue and then we're good.

[Copilot]

Pull request overview

Adds user-facing warnings around age passphrase keychain caching when the OS keyring is unavailable or when runtime keyring writes fail, and threads context.Context through the caching and askpass call paths to support those warnings.

Changes:

• Extend cacher.Set and askPass.Passphrase to accept context.Context, and update call sites.
• Emit a one-time warning when age.usekeychain is enabled but no OS keyring provider is available.
• Warn when writing a passphrase to the OS keyring fails, and add targeted tests for keyring/get/set + warning behavior.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
internal/cache/inmem.go	Updates in-memory cache Set signature to accept context.Context (unused) for interface compatibility.
internal/cache/inmem_test.go	Updates tests to pass t.Context() into Set.
internal/backend/crypto/age/keyring.go	Passes ctx into askPass.Passphrase within password callback wiring.
internal/backend/crypto/age/identities.go	Passes ctx into askPass.Passphrase in identity load/save flows.
internal/backend/crypto/age/decrypt.go	Passes ctx into askPass.Passphrase in decrypt fallback flow.
internal/backend/crypto/age/askpass.go	Adds one-time “keyring unavailable” warning, adds runtime keyring-write warning, and threads ctx into cache Set.
internal/backend/crypto/age/askpass_test.go	Adds tests covering askPass initialization, OS keyring get/set behavior, and user-facing warning output.

[Copilot]

osKeyring.Get currently uses o.knownKeys[name] = true (just above this return), but the parameter is named key. This will not compile and also prevents tracking retrieved keys. Use the key variable (or rename the parameter consistently) when updating knownKeys.

[Copilot]

debug.Log ultimately formats via fmt.Fprintf, where the %w verb isn’t supported (it will render as %!w(...)). Use %v/%s for logging errors here (and similarly in the OS keyring get/set log messages).