feat(env-injection): value filters on resolved references (#138)

[arimxyer]

Closes #138. Completes item 3 of the #115 env-injection epic.

Summary

A credential reference may end with | filter to transform the resolved value before injection. It's parsed in the one shared grammar, so it works everywhere a reference is accepted — ${pass:...} templates (inject, exec --env-file), --set (exec/export), and .pass-cli.toml manifests.

${pass:api/token | base64}     # base64 a Bearer token
${pass:api | basicauth}        # base64("username:password") for an HTTP Basic header
pass-cli exec --set 'AUTH=api|basicauth' -- ...

Filters (one per reference):

filter	result
base64	standard base64 of the value
base64url	URL-safe base64 of the value
basicauth	base64("username:password") from the credential (takes no field)

Design

• Parse once in the shared SplitPath (internal/envmap): peel an optional | filter, trim only when a pipe is present (filterless refs are byte-for-byte unchanged), validate the name fail-closed, reserve the first |. New 4-return signature propagated to ParseSetSpec, parseTemplate, ParseManifest.
• Carry via a Filter field on envmap.Mapping and TemplateRef.
• Apply client-side in resolver.ResolveValuesFiltered, after the backend returns raw values — so the agent's values-only protocol is untouched and output is identical in direct and daemon modes. basicauth resolves username + password through the normal (batched) value path and base64s user:pass.
• Single-source registry (internal/envmap/filter.go): IsKnownFilter (parse-time) and ApplyValueFilter (apply-time) both derive from one map, so they can't drift.

Intentional limitations (documented)

• The positional <service> form is never parsed by SplitPath, so filters apply only to --set, --from manifests, and ${pass:...}.
• On --set the | must be shell-quoted.
• Fail-closed: an unknown/empty filter is a hard error and nothing is injected; for --set/manifests this is caught before the vault is opened.

Test

• Unit: SplitPath filter parsing (spaces, legacy colon, empty/unknown/chaining/multi-segment errors, basicauth-no-field); the filter registry; ResolveValuesFiltered (base64 + basicauth + unknown propagation).
• Integration: inject (base64, basicauth, unknown-fails-closed), export --set (base64, proving filter-then-shell-quote order), and an agent-path case proving the filter is applied client-side while the daemon serves the raw value.

go build ✓ · go vet ./... ✓ · go test ./... ✓ · go test -tags=integration (filter set) ✓ · golangci-lint run ./... → 0 issues ✓

Out of scope (future)

Filter chaining (| trim | base64), base64url-nopad, a general two-reference join operator, and more filters (hex, upper/lower, trim).

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