feat: support Python 3.10-3.14, drop 3.8/3.9, update plugin compatibility (#386)

## Summary

Support Python 3.10 through 3.14, drop end-of-life Python 3.8 and 3.9. Update plugin `support_matrix` and test services for newer library versions on 3.12+.

Supersedes #374.

### Core agent fixes
- Fix missing `import importlib.util` that caused `NameError` on Python 3.12+ (root cause of #374 failures)
- Replace deprecated `find_module`/`load_module` with `find_spec`/`module_from_spec` in plugin loader and doc generator
- Remove Python <3.8 `pkg_resources` fallback (dead code)

### Dependency updates
- Python range: `>=3.10, <3.15`
- Unpin `psutil`, `packaging`; loosen `wrapt>=1.14`, `uvloop>=0.17`
- Remove `uwsgi` from dev deps (doesn't build on 3.12+)
- Mark plugins/lint groups as optional; use `>=` ranges so `poetry lock` resolves on all Python versions
- Upgrade pylint pin from `2.13.9` to `>=2.13.9`
- Remove Poetry 1.5.1 pin on Linux (can't read Poetry 2.x lock files)

### Plugin support_matrix updates for 3.12+
All hook points verified against new library versions:

| Plugin | >=3.10 | >=3.12 | >=3.13 | >=3.14 |
|--------|--------|--------|--------|--------|
| bottle | 0.12.23 | 0.12.23 | 0.13 | 0.13 |
| django | 3.2 | 3.2 | 5.1 | 5.1 |
| flask | 2.0 | 2.0 | 2.0 | 3.0 |
| tornado | 6.0, 6.1 | 6.0, 6.1 | 6.0, 6.1 | 6.4 |
| pyramid | 1.10, 2.0 | 2.1 | 2.1 | 2.1 |
| kafka-python | 2.0 | 2.3 | 2.3 | 2.3 |
| pulsar-client | 3.3.0 | 3.9.0 | 3.9.0 | 3.9.0 |
| psycopg | 3.0/3.1 | 3.1 | 3.2 | 3.2 |
| httpx | 0.22/0.23 | 0.22/0.23 | 0.23 | 0.28/0.23 |
| happybase | 1.2.0 | 1.3.0 | 1.3.0 | 1.3.0 |
| urllib3 | 1.25/1.26 | skipped | skipped | skipped |

- **urllib3**: skipped on 3.12+ — `urllib3.request.RequestMethods` was removed in urllib3 2.x, plugin needs code adaptation
- **happybase**: upgraded to 1.3.0 for 3.12+ (thriftpy2 0.6.0 has cp312/cp313 wheels)

### Test service fixes
- Django test services: replace removed `django.conf.urls.url` with `django.urls.path` (compatible with Django 2.0+ through 5.1)
- Fix `testcontainers` DockerCompose API for v4 compatibility (`filepath` -> `context`)

### CI updates
- Test matrix: Python 3.10/3.11/3.12/3.13/3.14 (dropped 3.8/3.9)
- Fix test step: `poetry install --without plugins,lint` (avoids installing incompatible old plugin deps on host)
- Replace unapproved `getsentry/paths-filter` with approved `dorny/paths-filter`
- Exclude `profiling_greenlet` E2E on 3.14 (gevent doesn't have 3.14 wheels yet)

### Docker & docs
- Update docker/Makefile for 3.10-3.14
- Update Container.md with version range and known limitations
- Add CLAUDE.md and Claude Code skills for plugin development

### Locally verified (with span data validation)
- django==5.1 on Python 3.13 — PASSED
- psycopg[binary]==3.2.* on Python 3.13 — PASSED  
- happybase==1.3.0 on Python 3.13 — PASSED
- httpx==0.23.* on Python 3.13 — PASSED
- requests==2.26/2.25 on Python 3.13 — PASSED
- Agent loads all 35 plugins on Python 3.10/3.11/3.12/3.13/3.14