feat(registry): workflow registry system — named registries, versioning, caching

[jrob5756]

Summary

Introduces a configurable named-registry system for distributing and running shared workflows. Supersedes #88 and addresses #78.

Full design doc: docs/design/registry.md

What's new

• Named registries — GitHub repos or local directories, configured in ~/.conductor/registries.toml
• Registry CLI — conductor registry {list,add,remove,set-default,update,show}
• Run from registry — conductor run qa-bot@official@1.2.3 (or just qa-bot for latest from default)
• Explicit versioning — npm-style, backed by git tags for GitHub registries
• Local caching — ~/.conductor/cache/registries/, explicit versions immutable, latest re-resolved on update
• Index manifest — index.yaml at registry root lists workflows, descriptions, versions, and paths

What's removed

• conductor init / conductor templates commands
• src/conductor/cli/init.py and src/conductor/templates/ directory

Changes

New: src/conductor/registry/ (6 modules)

Module	Purpose
config.py	Pydantic models for registries.toml, load/save/add/remove helpers
resolver.py	Parse name[@registry][@version], file-vs-registry detection
index.py	Load/parse index.yaml from GitHub or local path
cache.py	Local cache management, workflow + sibling file fetching
github.py	GitHub raw file fetch, tag listing, directory enumeration
errors.py	RegistryError extending ConductorError

Modified

• src/conductor/cli/app.py — run/resume/validate accept registry refs; registry subcommand wired in; init/templates removed
• pyproject.toml — added httpx>=0.27.0
• README, docs/cli-reference.md, conductor skill docs

Tests

• 133 new tests across tests/test_registry/ (unit + integration) and tests/test_cli/test_registry_commands.py
• All 1977 tests pass

Reference syntax

conductor run ./workflow.yaml          # local file (unchanged)
conductor run qa-bot                   # latest from default registry
conductor run qa-bot@team              # latest from named registry
conductor run qa-bot@team@1.2.3        # explicit version

Closes #78