docs: record exported API review and approval of E1-E6

[ajbeck]

Adds decision 0026, a review of every exported identifier in edge, identity, authn, iamproof and authz. The review is based on Google's Go style guide (guide, decisions and best practices, read in full) and the primary sources it links. This PR changes no code: it records proposals for owner review, as AGENTS.md requires for public API decisions.

Stacked on #5.

Proposals (each separately approvable)

ID	Recommendation	Breaking
E1	Move ActionHeader out of the root edge package into a standard-library-only actionheader package. Today native HTTP consumers link the Lambda adapter just to select an action.	Yes
E2	Rename the action sentinels so they match the module's adjective-first names (ErrInvalidEvent, ErrMissingCredentials)	Yes
E3	Rename identity.WithCaller to identity.NewContext, pairing with FromContext. Every other With… function in the module is an option.	Yes
E4	Typed constants for the InvocationError operation and limit names, which today are documented only in prose	Mostly compatible
E5	String methods on the identity enums (today Caller prints kind:2,source:4)	No
E6	Doc pass: per-sentinel comments (with HTTP status for authn), doc links, named callback parameters, Claim.Bool result names	No
E7	Move the Cognito verifier into authn/cognito (discretionary)	Yes

The record also lists what was considered and not recommended, with reasons. That includes keeping the Handler error return, the explicit iamproof error adapter, the shared edge.Option type and the current config styles. It also notes what is already conformant.

Validation

• Every claim was checked against the source code, go doc -all output and the accepted decision records. E1 reopens part of decision 0011; the record says so and asks the owner to confirm.
• All 34 cited style-guide and Go-doc anchors and all relative links resolve. The record renders correctly, and git diff --check is clean.
• No Go files changed, so the Go test suite was not rerun.

Limitations

• The resolution is pending: merging this PR records the proposals, not approval to implement them.
• Package names in E1 and E7 are suggestions and are listed as open questions.