[Feat]: Update supported-version policy / security

[rosspeili]

Feature Description

Retire the current three-band policy (< 0.2.6 unsupported, 0.2.6–0.3.0 no fixes, >= 0.3.1 supported) and align code + docs with:

Installed version	Security support	CLI advisory
>= 0.3.5	Supported — security reports accepted and patched	Silent
0.3.0 – 0.3.4	No security fixes; upgrade recommended	Silent
<= 0.2.9 (e.g. 0.2.9, 0.2.6)	Unsupported	One dim stderr message at CLI startup

Files to update

• skillware/version_policy.py — MIN_UNSUPPORTED, MIN_SECURITY_SUPPORTED, UPGRADE_TARGET (and any helper logic if a middle band needs explicit handling)
• SECURITY.md — supported-versions table
• tests/test_version_policy.py — threshold assertions and advisory message (>= 0.3.5)
• docs/usage/cli.md — Version advisory section (currently documents 0.2.6 / 0.3.1)

Note: pyproject.toml only holds the package release version (0.3.9); policy constants stay in version_policy.py unless maintainers decide to document them elsewhere.

Add CHANGELOG.md entry under [Unreleased] when merged.

Rationale

Project is on 0.3.9. The 0.3.1 security floor is outdated. Tightening bands reflects real support intent: patch only recent releases (0.3.5+), treat 0.3.0–0.3.4 as legacy without security backports, and warn only on end-of-life installs (0.2.9 and below).

Implementation Idea

In version_policy.py:

• MIN_UNSUPPORTED = Version("0.3.0") → advisory when installed < 0.3.0 (covers 0.2.9 and earlier)
• MIN_SECURITY_SUPPORTED = Version("0.3.5") → document in SECURITY.md; no CLI spam for 0.3.0–0.3.4
• UPGRADE_TARGET = "0.3.5"

Keep should_emit_unsupported_advisory() as installed < MIN_UNSUPPORTED only.

Update tests/test_version_policy.py for 0.2.9 (advisory), 0.3.0 / 0.3.4 (silent), 0.3.5+ (silent), and assert upgrade message references >=0.3.5.

Mirror the table in SECURITY.md and the Version advisory section in docs/usage/cli.md.