feat: add Okta SAML credential exchange with session cache

[youngjinjung-linq]

Summary

Implements #85 and #86 (Okta SAML track #26). Stacked on #244 — merge that first; this diff includes it until then.

• Exchange provider (internal/auth/okta.go): Okta primary authentication (/api/v1/authn) → one-time session token → SAML assertion scraped from the app embed link's auto-submit form → sts:AssumeRoleWithSAML with anonymous credentials.
• Credential input: prompt on stderr with no-echo password input (golang.org/x/term); UNIC_OKTA_USERNAME/UNIC_OKTA_PASSWORD env vars override prompting for automation. Passwords, session tokens, and MFA secrets are never persisted.
• Deterministic role selection: the context's role_arn wins; a single assertion role is auto-selected; multiple roles without a preference produce an explicit error listing the ARNs.
• Session cache (internal/services/aws/okta_saml_cache.go): exchanged AWS credentials cached under ~/.config/unic/cache/okta-saml/ (0700 dir / 0600 files, keyed by org|app|role hash) with a 2-minute expiry skew, mirroring the SSO and assume-role MFA cache patterns.
• Wiring: unic env resolves cached-or-fresh sessions; the TUI (NewAwsRepository) passively reuses a valid cached session and otherwise fails fast with run 'unic env <context>' first to sign in to Okta.
• Okta MFA_REQUIRED/MFA_ENROLL responses are rejected with a clear pointer to the upcoming feat: Add Okta MFA challenge flow for okta_saml #87 challenge flow.

Testing

• go test ./... passes: SAML role parsing (both role/provider orderings), role selection (preferred/single/multiple/missing), end-to-end resolve against an httptest Okta (success, MFA_REQUIRED, 401), cache reuse without prompting, cache round-trip/expiry, and repository cache-hit/cache-miss paths.
• make build passes.

Closes #85
Closes #86

Part of #26

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📥 Commits

Reviewing files that changed from the base of the PR and between 744b139 and 5796a99.

⛔ Files ignored due to path filters (1)

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📒 Files selected for processing (12)

• README.md
• go.mod
• internal/app/context_add.go
• internal/app/context_add_test.go
• internal/auth/env.go
• internal/auth/okta.go
• internal/auth/okta_test.go
• internal/config/config.go
• internal/config/config_test.go
• internal/services/aws/okta_saml_cache.go
• internal/services/aws/okta_saml_cache_test.go
• internal/services/aws/repository.go

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[YoungJinJung]

Recreated as #258 from an in-repo branch under the maintainer account; this PR is superseded.