refactor: introduce ConnectorRegistry to improve connector ecosystem extensibility and maintainability

[mtfang]

Summary

This PR delivers the highest-leverage parts of the Connector Ecosystem Refactor Plan:

• Centralized ConnectorRegistry + self-registration for built-in connectors in alvum-core
• Elimination of the hardcoded match name dispatch in the CLI (the main source of the "add a new connector = touch 6 files" problem)
• ConnectorBuildContext to cleanly handle the two per-run dependencies that aren't uniform across connectors (image_provider for screen vision, since/before for session importers)
• Extraction of small shared helpers (resolve_path, emit_connector_warning) that immediately delete duplication
• Solid test coverage (new core unit tests + pipeline integration test)

All four core connectors (audio, screen, claude-code, codex) plus the external HTTP extension path continue to work exactly as before.

Motivation

The connector ecosystem had become the highest-friction area for adding new data sources (health, git activity, calendar, browser history, wearables, etc.). The review identified:

• Hardcoded registration in extract.rs, capture.rs, config.rs, builtin_components.rs, and the connectors CLI
• Heavy boilerplate duplication in every Connector implementation
• A vestigial capture_sources() method on the trait that is not used for the real daemon path

This PR deletes the worst of that coupling.

Review

Developed and reviewed using the /implement --effort 5 skill:

• 3 independent general reviewers
• Tests specialist (coverage of new registry/factory paths)
• Security specialist (data flow for user-controlled settings and LlmProvider)
• Plan Alignment specialist (fidelity to the approved plan)

One high-severity correctness issue was caught by a general reviewer (unconditional with_since/with_before clobber when CLI flags were absent) and fixed before merge.

Verification

cargo fmt -p alvum-core -p alvum-cli -p alvum-pipeline -- --check
cargo clippy -p alvum-core -p alvum-cli -p alvum-pipeline -- -D warnings
cargo test -p alvum-core --lib
cargo test -p alvum-pipeline --test idempotency
cargo test -p alvum-connector-audio --test processor_modes

All green. Existing alvum extract behavior, warning events, resume/fingerprint logic, and external connector routing are unchanged.

Follow-up PRs (tracked in the plan)

• Remove capture_sources() from the Connector trait (it is vestigial for built-ins)
• Move the audio/screen settings bridging functions into the connector crates themselves
• Make alvum connectors list --json and the menubar Extensions view derive core connectors from the registry + builtin_components
• docs/superpowers/specs/connector-development.md + evergreen "why" comments

Files

See the diff for the 12 changed files (mostly concentrated in alvum-core and the four connector crates + the CLI entry point).

This is the foundation that makes the rest of the connector ecosystem work sustainable long-term.